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.

This bestowed enormous political and cultural power on the American left, and led to the greatest array of government-sponsored social programs in history—at an expense, by some estimates, of more than $22 trillion. But for the left to wield this power, there had to be a great menace to fight against—a tenacious menace that kept America uncertain of its legitimacy, afraid for its good name.

This amounted to a formula for power: The greater the menace to the nation’s moral legitimacy, the more power redounded to the left. And the ’60s handed the left a laundry list of menaces to be defeated. If racism was necessarily at the top of the list, it was quickly followed by a litany of bigotries ending in “ism” and “phobia.”

The left had important achievements. It did rescue America from an unsustainable moral illegitimacy. It also established the great menace of racism as America’s most intolerable disgrace. But the left’s success has plunged it into its greatest crisis since the ’60s. The Achilles’ heel of the left has been its dependence on menace for power. Think of all the things it can ask for in the name of fighting menaces like “systemic racism” and “structural inequality.” But what happens when the evils that menace us begin to fade, and then keep fading?

It is undeniable that America has achieved since the ’60s one of the greatest moral evolutions ever. That is a profound problem for the left, whose existence is threatened by the diminishment of racial oppression. The left’s unspoken terror is that racism is no longer menacing enough to support its own power. The great crisis for the left today—the source of its angst and hatefulness—is its own encroaching obsolescence. Today the left looks to be slowly dying from lack of racial menace.

A single white-on-black shooting in Ferguson, Mo., four years ago resulted in a prolonged media blitz and the involvement of the president of the United States. In that same four-year period, thousands of black-on-black shootings took place in Chicago, hometown of the then-president, yet they inspired very little media coverage and no serious presidential commentary.

White-on-black shootings evoke America’s history of racism and so carry an iconic payload of menace. Black-on-black shootings carry no such payload, although they are truly menacing to the black community. They evoke only despair. And the left gets power from fighting white evil, not black despair.

Today’s left lacks worthy menaces to fight. It is driven to find a replacement for racism, some sweeping historical wrongdoing that morally empowers those who oppose it. (Climate change?) Failing this, only hatred is left.

Hatred is a transformative power. It can make the innocuous into the menacing. So it has become a weapon of choice. The left has used hate to transform President Trump into a symbol of the new racism, not a flawed president but a systemic evil. And he must be opposed as one opposes racism, with a scorched-earth absolutism.

For Martin Luther King Jr., hatred was not necessary as a means to power. The actual details of oppression were enough. Power came to him because he rejected hate as a method of resisting menace. He called on blacks not to be defined by what menaced them. Today, because menace provides moral empowerment, blacks and their ostensible allies indulge in it. The menace of black victimization becomes the unarguable truth of the black identity. And here we are again, forever victims.

Yet the left is still stalked by obsolescence. There is simply not enough menace to service its demands for power. The voices that speak for the left have never been less convincing. It is hard for people to see the menace that drives millionaire football players to kneel before the flag. And then there is the failure of virtually every program the left has ever espoused—welfare, public housing, school busing, affirmative action, diversity programs, and so on.

For the American left today, the indulgence in hate is a death rattle.

Mr. Steele, a senior fellow at Stanford University’s Hoover Institution, is author of “Shame: How America’s Past Sins Have Polarized Our Country” (Basic Books, 2015).