LAS VEGAS — Donald J. Trump has shaken up his presidential campaign for the second time in two months, hiring a top executive from the conservative website Breitbart News and promoting a senior adviser in an effort to right his faltering campaign.

Stephen Bannon, the executive chairman of Breitbart News LLC, will become the Republican campaign’s chief executive, and Kellyanne Conway, a senior adviser and pollster for Mr. Trump and his running mate, Gov. Mike Pence of Indiana, will become the campaign manager.

Paul Manafort, the campaign chairman, will retain his title. But the staffing change, hammered out on Sunday and set to be formally announced Wednesday morning, was seen by some as a demotion for Mr. Manafort.

The news, first reported by The Wall Street Journal, was confirmed early Wednesday by Ms. Conway in a brief interview, but she rejected the idea that the changes amounted to a shake-up and said that Mr. Manafort was not being diminished.

“It’s an expansion at a busy time in the final stretch of the campaign,” she said, adding that Mr. Manafort and his deputy, Rick Gates, would remain in their roles.

“We met as the ‘core four’ today,” Ms. Conway added, referring to herself, Mr. Bannon, Mr. Manafort and Mr. Gates.

People briefed on the move said that it reflected Mr. Trump’s realization that his campaign was at a crisis point. But it indicates that the candidate — who has chafed at making the types of changes his current aides have asked for, even though he had acknowledged they would need to occur — has decided to embrace his aggressive style for the duration of the race.

Both Ms. Conway and Mr. Bannon, whose news organization has been very favorable to Mr. Trump since he entered the primaries, are close with Robert and Rebekah Mercer, the father-and-daughter conservative donors who have become allies of the candidate and are funding a “super PAC” that is working against Hillary Clinton.

Ms. Conway has past presidential experience in primary races, but the role in a general election represents a new one for her. She is well liked by Mr. Trump’s daughter Ivanka and her husband, Jared Kushner, who had been serving as the de facto campaign manager.

Mr. Bannon has no experience with political campaigns, but he represents the type of bare-knuckled fighter that the candidate had in Corey Lewandowski, his combative former campaign manager, who was fired on June 20.

Mr. Bannon has been a supporter of Mr. Trump’s pugilistic instincts, which the candidate has made clear in interviews he is uncertain about suppressing. He is also deeply mistrustful of the political establishment, and his website has often been critical of Speaker Paul D. Ryan and Senator Mitch McConnell, the majority leader.

Rudolph W. Giuliani, the former mayor of New York City who has become a close Trump adviser, has also urged the candidate to dig in and prepare to fight harder, and in a more focused way, in what has quickly become one of the nastiest presidential campaigns in modern United States history.

Mr. Manafort, who had initially been hired to steer Mr. Trump through what appeared to be a protracted fight for delegates, rose in power after repeated clashes with Mr. Lewandowski.

Mr. Lewandowski was ultimately fired with the help of Mr. Trump’s adult children, who believed the campaign manager was trying to spread negative stories about Mr. Kushner.

Mr. Lewandowski, now a paid CNN commentator, has denied that that was the case, and he and Mr. Trump still speak frequently, with the candidate seeking his advice.

Mr. Lewandowski’s troubles began, in part, when he was accused by a female Breitbart reporter, who worked for Mr. Bannon, of grabbing her roughly after a news conference at one of his Florida properties. He was charged with assault, but prosecutors declined to proceed with the case, which was dropped.

People briefed on the reshuffling were adamant that Mr. Trump’s children would seek to block a return by Mr. Lewandowski. And they insisted that staff departures resulting from the changes would be few.

The candidate has bucked efforts to rein in his impulsive behavior, committing repeated gaffes after telling his aides he planned to adopt a more presidential tone.