Trump, Lacking Clear Authority, Says U.S. Will Declare Antifa a Terrorist Group

President Trump said on Twitter on Sunday that the United States would designate a group of far-left anti-fascism activists as a terrorist organization, a declaration that lacked any clear legal authority, as his administration sought to blame the group for violent protests across the nation over the weekend.

“The United States of America will be designating ANTIFA as a Terrorist Organization,” Mr. Trump wrote.

The president has periodically criticized antifa, a contraction of the word “anti-fascist” that has come to be associated with a diffuse movement of left-wing protesters who engage in more aggressive techniques like vandalism.

But it was not clear that Mr. Trump’s declaration would have any real meaning beyond his characteristic attempts to stir a culture-war controversy, attract attention and please his conservative base.

First, antifa is not an organization. It does not have a leader, membership roles or any defined, centralized structure. Rather, it is a vaguely defined movement of people who share common protest tactics and targets.

More important, even if antifa were a real organization, the laws that permit the federal government to deem entities terrorists and impose sanctions on them are limited to foreign groups. There is no domestic terrorism law, despite periodic proposals to create one.

“There is no authority under law to do that — and if such a statute were passed, it would face serious First Amendment challenges,” said Mary B. McCord, a former head of the Justice Department’s National Security Division. “But right now, the only terrorist authority is for foreign terrorist organizations.”

In dealing with effectively domestic terrorism investigations into neo-Nazi organizations like the Base and Atomwaffen Division, for example, the F.B.I. has treated them as criminal enterprises.

Nevertheless, in a statement after Mr. Trump’s tweet, Attorney General William P. Barr said the F.B.I. would use its partnerships with state and local police to identify violent protesters, whom he also called domestic terrorists.

“The violence instigated and carried out by antifa and other similar groups in connection with the rioting is domestic terrorism and will be treated accordingly,” Mr. Barr said.

But the American Civil Liberties Union condemned Mr. Trump’s vow in a statement from Hina Shamsi, its national security project director.

“As this tweet demonstrates, terrorism is an inherently political label, easily abused and misused,” Ms. Shamsi said. “There is no legal authority for designating a domestic group. Any such designation would raise significant due process and First Amendment concerns.”

Mr. Trump’s tweet appeared to be part of an effort by his administration to blame far-left activists for the violence gripping the nation after the death of George Floyd in police custody in Minneapolis last week. Earlier Sunday, Mr. Trump’s national security adviser, Robert C. O’Brien, had blamed such activists during appearances on CNN and ABC News, saying that he had not seen anything to corroborate reports by the Department of Homeland Security and the news media that far-right groups were also stoking violence.

Mr. O’Brien said the F.B.I. needed to “come up with a plan” to deal with antifa.

“Right now, I think the president and the attorney general, Barr, want to know what the F.B.I.’s been doing to surveil, to disrupt, to take down antifa, to prosecute them,” Mr. O’Brien said on the ABC program “This Week.” “This isn’t the first time they’re out there. And they are using military-style tactics and traveling around the country to take advantage of these situations and burn down our cities.”

That Mr. Trump does not have the apparent legal authority to tell his administration to designate a group as a domestic terrorist organization did not matter to his supporters, several of whom praised his statement. One adviser to Mr. Trump said that appearing to crack down on the protests was politically important to the president, with just five months until Election Day.

Julian Barnes and Adam Goldman contributed reporting.