When did the movement begin?

Although the Merriam-Webster dictionary says the word “antifa” was first used in 1946 and was borrowed from a German phrase signaling an opposition to Nazism, more people began joining the movement in the United States after the 2016 election of Mr. Trump, to counter the threat they believed was posed by the so-called alt-right, Mr. Bray said.

One of the first groups in the United States to use the name was Rose City Antifa, which says it was founded in 2007 in Portland, Ore. It has a large following on social media, where it shares news articles and sometimes seeks to dox, or reveal the identities and personal information of, figures on the right.

The antifa movement gained more visibility in 2017 after a series of events that put a spotlight on anti-fascist protesters, including the punching of a prominent alt-right member; the cancellation of an event by a right-wing writer at the University of California, Berkeley; and their confrontation of white nationalist protesters in Charlottesville who turned violent.

What distinguishes antifa from other protest groups?

Mr. Bray said antifa groups often use tactics similar to anarchist groups, such as dressing in all black and wearing masks. The groups also have overlapping ideologies, as both often criticize capitalism and seek to dismantle structures of authority, including police forces.

How have politicians and others reacted?

The movement has been widely criticized among the mainstream left and right. After the protests in Berkeley, Calif., in August 2017, Speaker Nancy Pelosi decried “the violent actions of people calling themselves antifa” and said they should be arrested.

Conservative publications and politicians routinely rail against supporters of antifa, who they say are seeking to shut down peaceful expression of conservative views. These critics point to moments during which purported antifa members have been accused of sucker-punching Trump supporters.

Ruth Ben-Ghiat, a professor of history at New York University who studies fascism, said she was worried that antifa’s methods could feed into what she said were false equivalencies that seek to lump violence on the left with attacks by the right, such as the killing of a protester in Charlottesville by a man who had expressed white supremacist views.