Even before President Trump’s election, hatred had begun to emerge on the American left—counterintuitively, as an assertion of guilelessness and moral superiority. At the Women’s March in Washington the weekend after Mr. Trump’s inauguration, the pop star Madonna said, “I have thought an awful lot of blowing up the White House.” Here hatred was a vanity, a braggadocio meant to signal her innocence of the sort of evil that, in her mind, the White House represented. (She later said the comment was “taken wildly out of context.”)

For many on the left a hateful anti-Americanism has become a self-congratulatory lifestyle. “America was never that great,” New York Gov. Andrew Cuomo recently said. For radical groups like Black Lives Matter, hatred of America is a theme of identity, a display of racial pride.

For other leftists, hate is a license. Conservative speakers can be shouted down, even assaulted, on university campuses. Republican officials can be harassed in restaurants, in the street, in front of their homes. Certain leaders of the left—Rep. Maxine Waters comes to mind—are self-appointed practitioners of hate, urging their followers to think of hatred as power itself.

How did the American left—conceived to bring more compassion and justice to the world—become so given to hate? It began in the 1960s, when America finally accepted that slavery and segregation were profound moral failings. That acceptance changed America forever. It imposed a new moral imperative: America would have to show itself redeemed of these immoralities in order to stand as a legitimate democracy.

The genius of the left in the ’60s was simply to perceive the new moral imperative, and then to identify itself with it. Thus the labor of redeeming the nation from its immoral past would fall on the left. This is how the left put itself in charge of America’s moral legitimacy. The left, not the right—not conservatism—would set the terms of this legitimacy and deliver America from shame to decency.