THE LATEST symptom of America’s deepening political illness is the rise of “antifa” — short for “anti-fascist.” Clad in black and armed with clubs or pepper spray, these masked men and women style themselves the bane of neo-Nazis, Ku Klux Klansmen and other extreme rightists wherever the latter may appear. Unlike the use of force by the state, which antifa activists abhor as a matter of vague but intense ultra-left ideology, their violence is righteous — according to them.

Antifa and like-minded offshoots smashed windows and set fires in Washington on Inauguration Day, and committed such crimes again in Berkeley, Calif., soon after, to disrupt a planned speech by right-wing provocateur Milo Yiannopoulos. They were seen pummeling alleged right-wingers in Berkeley this past weekend, including a man they pushed to the ground and then kicked and punched until a journalist intervened. Whether the victim was or was not an actual fascist is not clear — but antifa’s indifference to such details is. “There is a complete mob mentality here,” the Los Angeles Times’s James Queally reported from the scene. “People are randomly accusing random people of being Nazis.”

Exactly what kind of threat does antifa pose? We would not for a minute equate it to the menace of violent, ultra-right white-supremacist groups, which are enjoying an ugly renaissance bred, in part, by the succor President Trump has given to racial and religious intolerance. Nor does the antifa movement pose a clear and present danger to the broader political system, though it has shown a disturbing capacity for intimidating and confusing various officials in locales from Middlebury, Vt., to the West Coast. Portland, Ore., canceled this year’s Rose Festival parade because antifa-linked groups threatened to attack marchers from the local Republican Party.

Rather, antifa’s true danger is twofold: First, its violence does obvious and unjustifiable harm, both to free speech and to people and property; second, it tends to discredit, through association, the far broader peaceful movement against racism and hate. That movement must win if the United States is to flourish, and it can win only by upholding democratic norms and the rule of law, even in the face of everything the ultra-right may do to undermine them.

Accordingly, “no enemies on the left” cannot be the rule, at least not as long as antifa is around. These people proudly are not liberals or democrats, much less liberal Democrats. To the contrary, if anyone should have qualms about the way police retreated when 100 antifa members showed up in Berkeley on Sunday — if anyone should be urging police forces across the country to deal firmly but appropriately with them — it’s the peaceful marchers against racism in that town, whose message was drowned out by the far more vivid violence of their ostensible defenders in black.

Mr. Trump’s equation of white supremacists in Charlottesville with those who rallied against them was false and repugnant, but antifa activists’ deeds hardly promote the moral clarity necessary to isolate right-wing hate groups. Over time, such violence only benefits the very forces antifa purports to oppose. In terms of objective political impact, the group is badly misnamed: “Profa” would be more accurate.