ST. LOUIS — House Speaker Paul D. Ryan (R-Wis.) told colleagues Monday he will no longer campaign for or defend GOP presidential nominee Donald Trump, even as Trump’s top advisers said he would keep up his offensive against Hillary Clinton.

In a conference call with GOP House members Monday morning, Ryan said he is “only campaigning for House seats and promoting our agenda,” according to multiple participants on the call who requested anonymity because of the sensitive nature of the topic.

A number of Republicans who have called on Trump to step aside have said he should make way for Mike Pence and some have said they plan on writing in Pence’s name when they cast their ballots.

Speaker Paul Ryan, R-Wisconsin, endorsed Trump in June and is among those Republicans who have condemned Trump, but haven’t withdrawn their support. On Friday night, however, Ryan disinvited him from an event they were scheduled to appear at together in Wisconsin.

RNC members, meanwhile, are scheduled to hold a 30-minute conference call in the early evening, according to Politico, which obtained an invitation. The conference call does not have a set agenda, the report said, but it follows the revolt from several dozen Republicans.

Donald J. Trump declared a $916 million loss on his 1995 income tax returns, a tax deduction so substantial it could have allowed him to legally avoid paying any federal income taxes for up to 18 years, records obtained by The New York Times show.

The 1995 tax records, never before disclosed, reveal the extraordinary tax benefits that Mr. Trump, the Republican presidential nominee, derived from the financial wreckage he left behind in the early 1990s through mismanagement of three Atlantic City casinos, his ill-fated foray into the airline business and his ill-timed purchase of the Plaza Hotel in Manhattan.

Tax experts hired by The Times to analyze Mr. Trump’s 1995 records said that tax rules especially advantageous to wealthy filers would have allowed Mr. Trump to use his $916 million loss to cancel out an equivalent amount of taxable income over an 18-year period.

Although Mr. Trump’s taxable income in subsequent years is as yet unknown, a $916 million loss in 1995 would have been large enough to wipe out more than $50 million a year in taxable income over 18 years.

The $916 million loss certainly could have eliminated any federal income taxes Mr. Trump otherwise would have owed on the $50,000 to $100,000 he was paid for each episode of “The Apprentice,” or the roughly $45 million he was paid between 1995 and 2009 when he was chairman or chief executive of the publicly traded company he created to assume ownership of his troubled Atlantic City casinos. Ordinary investors in the new company, meanwhile, saw the value of their shares plunge to 17 cents from $35.50, while scores of contractors went unpaid for work on Mr. Trump’s casinos and casino bondholders received pennies on the dollar.

“He has a vast benefit from his destruction” in the early 1990s, said one of the experts, Joel Rosenfeld, an assistant professor at New York University’s Schack Institute of Real Estate. Mr. Rosenfeld offered this description of what he would advise a client who came to him with a tax return like Mr. Trump’s: “Do you realize you can create $916 million in income without paying a nickel in taxes?”

Mr. Trump declined to comment on the documents. Instead, the campaign released a statement that neither challenged nor confirmed the $916 million loss.

“Mr. Trump is a highly-skilled businessman who has a fiduciary responsibility to his business, his family and his employees to pay no more tax than legally required,” the statement said. “That being said, Mr. Trump has paid hundreds of millions of dollars in property taxes, sales and excise taxes, real estate taxes, city taxes, state taxes, employee taxes and federal taxes.”

The statement continued, “Mr. Trump knows the tax code far better than anyone who has ever run for President and he is the only one that knows how to fix it.”

RESULTS: The proportion of pediatricians reporting parental vaccine refusals increased from 74.5% in 2006 to 87.0% in 2013 (P < .001). Pediatricians perceive that parents are increasingly refusing vaccinations because parents believe they are unnecessary (63.4% in 2006 vs 73.1% in 2013; P = .002). A total of 75.0% of pediatricians reported that parents delay vaccines because of concern about discomfort, and 72.5% indicated that they delay because of concern for immune system burden. In 2006, 6.1% of pediatricians reported “always” dismissing patients for continued vaccine refusal, and by 2013 that percentage increased to 11.7% (P = .004).

CONCLUSIONS: Pediatricians reported increased vaccine refusal between 2006 and 2013. They perceive that vaccine-refusing parents increasingly believe that immunizations are unnecessary. Pediatricians continue to provide vaccine education but are also dismissing patients at higher rates.

The mother of the U.S. ambassador killed in the 2012 Benghazi attacks called on Republicans to stop invoking her son on Friday, blasting their references to his death as “opportunistic and cynical.”

“As Ambassador J. Christopher Stevens’s mother, I am writing to object to any mention of his name and death in Benghazi, Libya, by Donald Trump’s campaign and the Republican Party,” Mary F. Commanday wrote in a letter to the editor published in the New York Times Friday.

“I know for certain that Chris would not have wanted his name or memory used in that connection,” she continued. “I hope that there will be an immediate and permanent stop to this opportunistic and cynical use by the campaign.”

The GOP made the attacks one of the major themes of its convention last week, pointing to the tragedy as evidence against Hillary Clinton's presidential campaign. Clinton was secretary of State at the time.

Another victim’s mother, Patricia Smith, spoke before the crowd of delegates on Monday, saying she blamed Democratic presumptive nominee for the death of her son, Sean Smith.

Hillary Clinton named Senator Tim Kaine of Virginia to be her running mate Friday, selecting a battleground state politician with working-class roots and a fluency in Spanish, traits that she believes can bolster her chances to defeat Donald J. Trump in November.

Mrs. Clinton’s choice, which she announced via text message to supporters, came after her advisers spent months poring over potential vice-presidential candidates who could lift the Democratic ticket in an unpredictable race against Mr. Trump.

In the end, Mrs. Clinton decided Mr. Kaine, 58, a former governor of Virginia who sits on the Senate Foreign Relations Committee and speaks fluent Spanish, had the qualifications and background and the personal chemistry with her to make the ticket a success.

Mrs. Clinton had entertained more daring choices. She considered Thomas E. Perez, the secretary of labor, who would have been the first Hispanic on a major party ticket; Senator Cory Booker, of New Jersey, who would have been the first African-American to seek the vice presidency; and Adm. James G. Stavridis, a retired four-star Navy admiral who served as the supreme allied commander at NATO, but had never held elected office.

In the end, Mrs. Clinton, who told PBS she is “afflicted with the responsibility gene,” avoided taking a chance with a less experienced vice-presidential candidate and felt no political need to push the historic nature of her candidacy by adding another woman or a minority to the ticket.

Mrs. Clinton and Mr. Kaine have similar centrist positions on foreign policy, education and criminal justice and they are said to share an easy rapport and a love of granular policy-making. “I do have a fondness for wonks,” Mrs. Clinton said in the PBS interview.

Asked whether Mr. Kaine was boring, Mrs. Clinton said “I love that about him.” She added, “He’s never lost an election.”

At a campaign stop with Mrs. Clinton in Annandale, Va., last week, Mr. Kaine tried out for the role. “Do you want a ‘You’re fired’ president or a ‘You’re hired’ president?” he asked the crowd. “Do you want a trash-talking president or a bridge-building president?” He compared Mrs. Clinton’s record of public service to that of his wife, Anne Holton, Virginia’s secretary of education. In recent days, President Bill Clinton and the White House had expressed support for Mr. Kaine.

Known as a pragmatic governor and senator, Mr. Kaine could help Mrs. Clinton appeal to independent voters and moderate Republicans displeased with Mr. Trump. But he could also turn off some liberal Democrats with his support of free trade agreements, which Senator Bernie Sanders of Vermont criticized to great effect in his nominating fight against Mrs. Clinton.

The son of a welder who owned a small metalworking shop, Mr. Kaine, a Roman Catholic, grew up in the Kansas City area. He attended a Jesuit boarding school and took a break from law school at Harvard to spend time as a Catholic missionary in Honduras, an experience that his family has said shaped him and helped him become fluent in Spanish.

Mrs. Clinton is expected to formally introduce Mr. Kaine as her running mate during a campaign swing through Florida, either at a rally at the state fairgrounds in Tampa on Friday or on Saturday at Florida International University in Miami, which has a large number of Hispanic students.

Mr. Kaine worked on fair housing and civil right issues as a lawyer. He was elected to the city council in Richmond, Va., in 1994, then proceeded to climb the ranks of elected office in the state. He became city’s mayor in 1998, the state’s lieutenant governor in 2002 and its governor in 2006. He also served as chairman of the Democratic National Committee.

As governor, Mr. Kaine drew some support from rural parts of the state as well strong backing in the state’s Democratic-leaning suburban areas. He led the state through one of its darkest times, the shooting at Virginia Tech that killed 32 people in 2007. In 2013, Mr. Kaine implored the United States Senate to find a “small measure of courage” to fight the gun lobby and impose tougher background checks on gun ownership.

Mr. Kaine was an early endorser of Senator Barack Obama’s presidential bid in the 2008 nominating fight against Mrs. Clinton. Mr. Kaine was also considered on Mr. Obama’s short list of vice-presidential candidates before Mr,. Obama selected Senator Joseph R. Biden Jr. of Delaware. In 2012, Mr. Kaine defeated George Allen, a Republican, to take the Senate seat being vacated by the Democrat Jim Webb.

Mrs. Clinton’s choice of Mr. Kaine underscores the changing demographics of Virginia, with its growing urban and minority populations.

Mr. Obama defeated John McCain in the state by more than six percentage points, the first time since Lyndon B. Johnson’s victory in 1964 that the state had voted for a Democratic presidential nominee. An NBC News-Wall Street Journal poll from July 15 shows Mrs. Clinton ahead of Mr. Trump for the state’s 13 electoral votes by nine percentage points.

With just days remaining before her announcement of a running mate, Mrs. Clinton had not yet made up her mind as her advisers debated what attributes voters might want in a vice president. She told Mr. Kaine and the other candidates of her decision in a round of phone calls on Friday. Mrs. Clinton came to the process with a unique vantage point, having been closely involved in her husband’s selection of then-Senator Al Gore of Tennessee in 1992, a choice that brought youth and Southern charm to a ticket already overflowing with it.

Mr. Trump, in contrast, had different needs. He selected Gov. Mike Pence of Indiana as his running mate, selecting a man with whom he has had little prior relationship. Mr. Pence, however, was chosen to help the New York businessman win the support of far-right conservatives who are skeptical of Mr. Trump’s social positions.

Mrs. Clinton has called Mr. Pence the “most extreme pick in a generation,” highlighting his positions against same-sex marriage and abortion rights and his support for prayer in the schools.

Indeed, Pence has been going through the motions of a potential running mate. He and his wife met with Trump earlier this month at the Trump National Golf Club in Bedminster, N.J., where the two men also played a round of golf.

“Spent time with Indiana Governor Mike Pence and family yesterday,” Trump tweeted afterward. “Very impressed, great people!”

Trump is expected to announce his decision this week, with the national convention set to commence Monday in Cleveland. Pence, meanwhile, faces a Friday deadline to withdraw from the Indiana ballot seeking re-election.

Still, an element of suspense remains. Trump has said he values unpredictability, and his VP preferences have seemed to shift on almost a daily basis. Still, a short list has emerged; Gingrich and Flynn have been in the conversation since early May.

But Gingrich and Pence have appeared to crystallize in recent days as the strongest potential candidates, judging by remarks made by Trump’s aides or the candidate himself. Christie does not have Washington political experience, one Trump ally noted; and, if Trump were to select a governor as his running mate, Pence’s record in Indiana would be easier to defend.

Although Gingrich or Christie might be a more ferocious attack dog than Pence, Trump has already assumed this role himself, perhaps opening the door for a more low-key running mate.

Pence, of course, has his own potential pitfalls, including the national backlash he incurred last year over a law that would have allowed Indiana businesses to deny services to gay and lesbian couples. He later changed the law in response to the outcry.

Trump’s campaign has been assessing these pros and cons during a formal vetting process, for which Pence has reportedly submitted documents.

Still, as the chatter about Pence has grown louder, he has nevertheless continued to run his own campaign for re-election. He stopped by the Bartholomew County 4-H Fair on Monday, where he toured barns filled with cattle and swine, according to local reports.

But Indiana Republicans are preparing for Pence to take on another campaign.

Said Seat, “Everyone here is operating as if this is happening.”

When I reported on the update last week, the description on the announcement read: "Every new title published from Microsoft Studios will support Xbox Play Anywhere ..."

Since then, a subtle difference has crept in. Can you spot it? "Every new title published from Microsoft Studios that we showed onstage at E3 this year will support Xbox Play Anywhere ..."

How do you predict a general election with Donald Trump?

We can think of a few basic approaches. One of them is to assert that precedent doesn’t apply to this election and that Trump’s case is sui generis. It’s not clear where that leads you, however.

If Trump is “unpredictable,” a phrase we heard used to describe him so often during the primaries, does that mean his chances of defeating Hillary Clinton are 50/50? If that’s what you think, you have the opportunity to make a highly profitable wager. Betting markets put Trump’s chances at only 20 percent to 25 percent instead.

In fact, despite (or perhaps because of) the unusual nature of his candidacy, the conventional wisdom holds that Trump is a fairly substantial underdog. In contrast to 2012, when there were frequent arguments over how solid President Obama’s lead in the polls was, there hasn’t been much of a conflict between “data journalists” and “traditional journalists” on this question of Trump’s chances. Nor has there been one between professionals who cover the campaign and the public; most experts expect Trump to lose, but so do most voters.

But should this seeming consensus give us more confidence — or make us nervous that we’re underestimating Trump again?

Giving Trump a 20 percent or 25 percent chance of becoming president means that Clinton has a 75 percent to 80 percent chance. That might seem generous given that, under ordinary circumstances, the background conditions of this election (no incumbent running and a mediocre economy) would seem to suggest a tossup. Are Clinton’s high odds justified on the basis of the polls? Or do they require making heroic assumptions about Trump, the same ones that got everyone, emphatically including yours truly, in trouble during the primaries?

The short answer is that 20 percent or 25 percent is a pretty reasonable estimate of Trump’s chances based on the polls and other empirical evidence. In fact, that’s quite close to where FiveThirtyEight’s statistical models, which are launching today, have the race. Our polls-only model has Trump with a 19 percent chance of beating Clinton as of early Wednesday afternoon. (The forecasts will continually update as new polls are added.) Our polls-plus model, which considers economic conditions along with the polls, is more optimistic about Trump, giving him a 26 percent chance.

Still, Trump faces longer odds and a bigger polling deficit than John McCain and Mitt Romney did at the same point in their respective races. He needs to look back to 1988 for comfort, when George H.W. Bush overcame a similar deficit against Michael Dukakis to win. Our models are built from data since 1972, so the probabilities we list account for elections such as 1980, 1988 and 1992, when the polls swung fairly wildly, along with others, such as 2004 and 2012, where the polls were quite stable.

If the middling economy is one silver lining for Trump, another is his swing state polls, which don’t seem to be as bad for him as his national polls. They aren’t good by any means, either, but whereas Trump trails Clinton by 6.7 percentage points in our average of national polls, according to our polls-only model, he’s down 4.8 points in our adjusted polling average of Ohio, 5.7 points in Florida, 3.9 points in Iowa, and 2.0 points in Colorado, for instance.

Again, we don’t mean to suggest that these are great numbers for Trump; the Florida result, for example, would represent the worst loss by a Republican there since 1948. Nonetheless, and somewhat in contrast to the conventional wisdom, our model suggests that Trump is more likely to win the Electoral College while losing the popular vote than the other way around. (Though the chances of either scenario are small.)

Some of this may be because we just haven’t had all that much swing state polling; it’s possible we’ll see leads for Clinton in the mid- to high single digits as these states are polled more often. Just this morning, for example, the firm Evolving Strategies published a set of polls in swing states showing Clinton leading Trump by 10 percentage points, on average. If there are more numbers like those, the model will adjust accordingly.

But there’s another potential explanation, which is that Trump is badly underperforming in red states, presumably as a result of having failed to consolidate the Republican base. That may put some traditionally red states into play for Clinton. For instance, Arizona, Missouri, North Carolina and the 2nd Congressional District of Nebraska4 are all tossups, according to the polls-only model. (Polls-plus has Trump narrowly favored in these places.)

Some of these states could be useful to Clinton. Arizona, in particular, could help Clinton put together some winning maps based on Western or heavily Hispanic states, even if she loses much of the industrial Midwest. Others, such as Missouri, are probably more superfluous. They could potentially add to Clinton’s Electoral College margin, but they aren’t likely to be tipping-point states that make the difference between her winning and losing.

That goes doubly for states such as Texas, Utah, Kansas and Alaska, where polls have often shown a single-digit margin for Trump and have occasionally even had Clinton winning. Republicans are used to racking up huge numbers of votes in these states, bolstering their standing in the national popular vote. If Trump wins Texas by only 6 percentage points instead of 16, that will hurt his popular-vote margin without affecting his Electoral College odds much.

Is the reverse also true? Is Trump overperforming in blue states, relative to how a Republican usually does? It depends on where you look. The Northeast was Trump’s strongest region in the primaries, and he’s gotten relatively good numbers — although he still trails Clinton — in polls of New Jersey, Connecticut and Maine. (He also leads Clinton in one poll of Maine’s 2nd Congressional District, which would be worth one electoral vote.5) However, he’s losing by typical margins in New York and California, where he has vowed to compete.

Overall, the polls so far suggest a slightly less polarized electorate and a somewhat wider playing field than we’ve gotten used to in recent years. That’s a potentially refreshing change, although it may prove to be ephemeral as both Clinton and Trump have room to grow with their party bases and could gain ground in traditionally blue and red states as a result.

About 20 people were killed and at least 42 people were wounded when a gunman armed with an assault-style rifle, a handgun and possibly an explosive device opened fire inside a gay nightclub in Orlando, Fla., about 2 a.m., Police Chief John Mina said.

The police classified the attack as a “terror incident.” Local and federal authorities were investigating, and the F.B.I. set up a hotline for tips.

The police and the federal authorities said they were pursuing several angles in the terrorism investigation, both overseas and in the United States.

The gunman was not from the Orlando area and was organized and well-prepared, the police said.

The gunman was found dead inside the Pulse nightclub, one of the largest gay clubs in Orlando, about 5 a.m. after a shootout with officers who were attempting to rescue dozens of people who were being held hostage, said Chief Mina. He confirmed that the gunman had been killed by the police.

The chief said it was difficult to assess the number of clubgoers who had been killed because many of the bodies had not been removed.

The police were searching the club for other explosives that the gunman may have planted during his assault on Pulse.

The shooting began inside the club, the chief said, and continued outside when an officer working at Pulse attempted to confront him. The gunman then ran back into the club, resumed shooting and took hostages.

At 5 a.m., Chief Mina said, the police decided to attempt a rescue, detonating two explosives to distract the gunman and to help clear the club.

“With that advantage,” the chief said, nine officers moved into the club to confront the gunman. In the ensuing shootout, one of the officers was slightly wounded, saved by his Kevlar helmet, the authorities said.

“We rescued about 30 people,” Chief Mina said. During the rescue and shootout, officers with the SWAT team — using an armored vehicle — entered the club and hurried people to safety. Many of them had sought shelter in other sections of the sprawling club.

The survivors escaped under the cover of what the police called the two “discretionary explosions,” the authorities said.

The wounded were taken to three area hospitals. Most were taken to Orlando Regional Medical Center, the area’s chief trauma center, which is just three blocks from the club.

On its Twitter feed, the Police Department asked other residents to “stay away from area” and said that it was seeking support from local state and federal agencies. Later, employees in the area were asked to stay home if possible.

The club itself posted a message on its Facebook page about 3 a.m.: “Everyone get out of pulse and keep running.”

Customers streamed out of the club into a chaotic situation with little idea of where to go. “Cops were saying, ‘Go, go, clear the area,’ ” Christopher Hansen told an Orlando television station. “You don’t know who’s what and who’s where.”

As the sun came up, helicopter-shot video showed a situation that seemed to be under control, although windows on one side of the club appeared to be shot out.

Phone camera videos shot at the scene show police cars, ambulances and other emergency vehicles outside the club on South Orange Avenue in downtown Orlando. Streets around the the club were shut down for several blocks.

Dozens of officers, paramedics and firefighters from the Orlando police, the Orange County Sheriff’s Office, the F.B.I. and three area fire departments were called to the scene.

“This will have a lasting effect on our community,” Chief Mina said. “We are a strong community.”

In the hours immediately following the shooting, the local television stations broadcast camera phone videos that captured the chaos at and near Pulse.

One of the videos clearly captured the rescue attempt. Shots could be heard as men and women ran from the nightclub.

Some of the clubgoers told local television reporters that the music was so loud it was difficult, at first, to distinguish the shots from the pulsing beat.

More than 100 people were believed inside the club when the shooting began about 2 a.m.

Orlando Regional Medical Center was placed on lockdown after receiving the first of the victims. “Only essential workers are being allowed access into the building,” the hospital said in a statement.

“Out of an abundance of caution, Arnold Palmer Hospital and Winnie Palmer Hospital have also been placed on lockdown,” the statement added.

On Pulse’s Facebook page, club officials promised to post updates as they came in: “As soon as we have any information, we will update everyone.”

“Please keep everyone in your prayers as we work through this tragic event,” the nightclub post said. “Thank you for your thoughts and love.”

Officials at Orlando Regional Medical Center asked family members of victims and missing people to gather at the north entrance, where they would be escorted inside.

Local television stations broadcast interviews with people outside the club and the hospital, anxiously waiting to hear from people who they said were still inside.

WKMG and WESH broadcast interviews with the crowds milling outside the hospital. One woman said she rushed to the area after she got a call from her daughter, who said, “Please come get us and come get us now.”

One mother told CNN: “A girlfriend of my son called and said he got shot, he got shot and she was crying.”

She said she could not find her son, or even get information on where he had been taken. Some of the victims had been transferred to other hospitals “because this hospital was full,” she said outside Orlando Regional Medical Center.

The club, which calls itself “Orlando’s Latin Hotspot,” hosts an “Upscale Latin Saturdays” party on Saturday night with three D.J.s and a midnight show.