Social conservatives. Libertarians. Country-club conservatives. Tea party conservatives. Everybody in politics knows that those sets of people who usually vote Republican cannot be arrayed in a continuum from moderately conservative to extremely conservative. They are on different political planes. They usually have just enough in common to vote for the same candidate.

Why then do we still talk about the left in terms of a continuum from moderately liberal to extremely liberal? Divisions have been occurring on the left that mirror the divisions on the right. Different segments of the left are now on different planes.

A few weeks ago, I was thrown into a situation where I shared drinks and dinner with two men who have held high positions in Democratic administrations. Both men are lifelong liberals. There's nothing "moderate" about their liberalism. But as the pleasant evening wore on (we knew that there was no point in trying to change anyone's opinion on anything), I was struck by how little their politics have to do with other elements of the left.

Their liberalism has nothing in common with the political mind-set that wants right-of-center speakers kept off college campuses, rationalizes the forced resignation of a CEO who opposes gay marriage, or thinks George F. Will should be fired for writing a column disagreeable to that mind-set. It has nothing to do with executive orders unilaterally disregarding large chunks of legislation signed into law or with using the IRS as a political weapon. My companions are on a different political plane from those on the left with that outlook—the progressive mind-set.

Wait, doesn't "progressive" today reflect the spirit of the Progressive Era a century ago, when the country benefited from the righteous efforts of muckrakers and others who fought big-city political bosses, attacked business monopolies and promoted Good Government?
The era was partly about that. But philosophically, the progressive movement at the turn of the 20th century had roots in German philosophy ( Hegel and Nietzsche were big favorites) and German public administration ( Woodrow Wilson's open reverence for Bismarck was typical among progressives). To simplify, progressive intellectuals were passionate advocates of rule by disinterested experts led by a strong unifying leader. They were in favor of using the state to mold social institutions in the interests of the collective. They thought that individualism and the Constitution were both outmoded.

That's not a description that Woodrow Wilson or the other leading progressive intellectuals would have argued with. They openly said it themselves.

It is that core philosophy extolling the urge to mold society that still animates progressives today—a mind-set that produces the shutdown of debate and growing intolerance that we are witnessing in today's America. Such thinking on the left also is behind the rationales for indulging President Obama in his anti-Constitutional use of executive power. If you want substantiation for what I'm saying, read Jonah Goldberg's 2008 book "Liberal Fascism," an erudite and closely argued exposition of American progressivism and its subsequent effects on liberalism. The title is all too accurate.

Here, I want to make a simple point about millions of people—like my liberal-minded dinner companions—who regularly vote Democratic and who are caught between a rock and a hard place.

Along with its intellectual legacy, the Progressive Era had a political legacy that corresponds to the liberalism of these millions of Democrats. They think that an activist federal government is a force for good, approve of the growing welfare state and hate the idea of publicly agreeing with a Republican about anything. But they also don't like the idea of shouting down anyone who disagrees with them.

They gave money to the ACLU in 1978 when the organization's absolutism on free speech led it to defend the right of neo-Nazis to march in Skokie, Ill. They still believe that the individual should not be sacrificed to the collective and that people who achieve honest success should be celebrated for what they have built. I'm not happy that they like the idea of a "living Constitution"—one that can be subjected to interpretations according to changing times—but they still believe in the separation of powers, checks and balances, and the president's duty to execute the laws faithfully.

These Democrats should get exclusive possession of the word "liberal."

As a libertarian, I am reluctant to give up the word "liberal." It used to refer to laissez-faire economics and limited government. But since libertarians aren't ever going to be able to retrieve its original meaning, we should start using "liberal" to designate the good guys on the left, reserving "progressive" for those who are enthusiastic about an unrestrained regulatory state, who think it's just fine to subordinate the interests of individuals to large social projects, who cheer the president's abuse of executive power and who have no problem rationalizing the stifling of dissent.

Making a clear distinction between liberals and progressives will help break down a Manichaean view of politics that afflicts the nation. Too many of us see those on the other side as not just misguided but evil. The solution is not a generalized "Can't we all just get along" non-judgmentalism. Some political differences are too great for that.

But liberalism as I want to use the term encompasses a set of views that can be held by people who care as much about America's exceptional heritage as I do. Conservatives' philosophical separation from that kind of liberalism is not much wider than the philosophical separation among the various elements of the right. If people from different political planes on the right can talk to each other, as they do all the time, so should they be able to talk to people on the liberal left, if we start making a distinction between liberalism and progressivism. To make that distinction is not semantic, but a way of realistically segmenting the alterations to the political landscape that the past half-century has brought us.