At lunch the other day, a friend and strong anti-Trumper wondered aloud what brought all those thousands of people out to Donald Trump’s rallies. “After all,” he said “they’re pretty much the same show.” Mr. Trump on stage, in his usual bragging mode, attacking the press, settling scores with people he feels have betrayed him, while the audience in their red hats applaud uproariously, yelling approval for 90 or so minutes. “What’s the attraction? I don’t get it.”

Not a bad question, really. As I thought it over, it occurred to me that what genuinely excites Mr. Trump’s crowds and draws them to him is their shared antiliberalism. By liberalism I do not mean liberalism of the kind that was at the center of our fathers’ Democratic Party—which supported labor unions, civil liberties, racial integration, involvement in international affairs. I refer to the liberalism now metamorphisized into progressivism, at the heart of the thinking of such Democrats as Elizabeth Warren, Bernie Sanders, Cory Booker, Kamala Harris, Alexandria Ocasio-Cortez and others.

This is the progressivism that edges into socialism, that is said to attract the young, that promises a newer, kinder America—the progressivism that exalts identity politics and has no argument with political correctness. As one looks upon the people who attend Mr. Trump’s rallies, one sees the faces not of Hillary Clinton’s “deplorables” but of the proletariat out of which Karl Marx’s dictatorship was supposed to derive. Yet these people, despite the progressives’ promises to them of free Medicare, free college tuition, and the rest, want nothing to do with Sens. Warren, Sanders, Booker & Co. Quite the reverse: They loathe them.

The man who attends a Trump rally turns on his television set and that night’s news leads off with a Black Lives Matter protest in his city. If that city is Chicago, he might recall that this year some 2,619 people have been shot, 475 shot and killed, the preponderance of these being black people shot by black youth gangs. If it is another city, there is a distinct possibility, as fairly often in the past, that the protest will lead to looting of nearby shops. Al Sharpton, nattily turned out, is likely to have flown in for the festivities to remind everyone about the world’s injustice.

Our man changes channels and is greeted by a story of a long and happy lesbian marriage. He reads in the papers that people are fired from jobs for remarks that, under the reign of political correctness, are interpreted as racist, sexist, you name it; that students feel unsafe at Yale; that a year’s tuition, room and board at Dartmouth is $74,000. Doubtless before long he will read a story about an 11-year-old who is suing his parents for not allowing him to transgender himself.

Oh God, he thinks, make America great again, make America straight again, make America anything but what it is becoming. What elected Donald Trump, and what sustains him, is not his rather dubious charisma, his ideas, his obvious jolt to the country’s earlier slow economic growth, and no, not even the wretched campaign run by Hillary Clinton. Mr. Trump was chosen as a rebuke to the progressivism that has made life in America seem chaotic, if not a touch mad, and that now threatens to take over the Democratic Party.

A number of highly intelligent Trump supporters I know are perfectly willing to acknowledge the president’s manifold flaws. They voted for him, and probably will again, because he is not Hillary or Barack Obama or Chuck Schumer. In the old Indian proverb, the enemy of their enemy is their friend; more than friend, he, Donald J. Trump, is happily their president.

After the midterm elections, Nancy Pelosi announced that she is exultant that more than 60% of the Democratic members of Congress are women, or ethic minorities, or LGBT. Some Democrats threaten to investigate President Trump’s personal finances. Others hold out the promise of impeaching Mr. Trump, Brett Kavanaugh or Matthew Whitaker. So the beat of identity politics, and progressivism generally, goes on.

The pull to the left of the Democratic Party is Donald Trump’s greatest hope for re-election, while Mr. Trump’s behavior is the greatest force pulling Democrats still further to the left. Tariffs, trade agreements, even immigration policy seem slightly beside the point when, as now, not two different parties but two radically different views of the good life dominate public discourse. And so things go, two ends without a middle. The shame is that most Americans find themselves in that missing middle, helpless without a party, hopeless without a leader. Politics has rarely seemed so dismal.

Mr. Epstein is author of “The Ideal of Culture and Other Essays” (Axios Press) and “Charm: The Elusive Enchantment” (Lyons Press).