During his speech in Phoenix on Tuesday night, Donald Trump spat out the nickname “antifa,” short for “anti-fascist” but also a reference to a particular strand of aggressive left-wing activism. In Mr. Trump’s telling, the presence of antifa activists during the violence in Charlottesville, Va., this month was evidence that the far left is just as violent as the far right: “You know, they show up in the helmets and the black masks, and they’ve got clubs and they’ve got everything.”

Surrogates have aped Mr. Trump’s “blame both sides” rhetoric; overnight, antifa — and its assumed synonym, “alt-left” — have become right-wing shibboleths, right there with “social justice warrior” and “liberal snowflake.” In truth, there is no symmetry between either “alt-right” and either “antifa” or “alt-left.” Antifa is the backlash to the backlash, a defensive response to the growing presence of right-wing extremism.

But that doesn’t mean we shouldn’t worry. Because antifa groups are willing to use force when needed, provoking them can trigger a self-fulfilling prophecy.

So far, there is a fearful asymmetry between the far right and antifa: Over the decade ending in 2016, estimates of the percentage of politically motivated killings committed by right-wing extremists range from 73 to 92 percent, according to the conservative Daily Caller. Despite the spurious rhetoric of equivalency, supporters of antifa have, to date, killed no one.