President Donald Trump said he plans to designate the left-wing ideology known as antifa as a “major terrorist organization,” an escalation of rhetoric against his political opponents amid heightened anxiety about polarization following the killing of conservative activist Charlie Kirk.

Antifa, shorthand for anti-fascists, is an umbrella description for the far-left-leaning militant groups that resist neo-Nazis and white supremacists at demonstrations and other events. It has long been criticized by Trump but he heightened his attacks in a social media post Wednesday while he was on a state visit to the U.K.

“I am pleased to inform our many U.S.A. Patriots that I am designating ANTIFA, A SICK, DANGEROUS, RADICAL LEFT DISASTER, AS A MAJOR TERRORIST ORGANIZATION,” Trump wrote on X. “I will also be strongly recommending that those funding ANTIFA be thoroughly investigated in accordance with the highest legal standards and practices.”

Trump has previously pledged to designate antifa as a terrorist organization, without detailing the mechanism by which he’d enforce the sanction. Trump made a similar threat amid nationwide protests following the death of George Floyd in 2020.

It’s unclear whether it’s possible for the president to label antifa as a terrorist organization. Former FBI Director Christopher Wray said antifa is “a movement or an ideology,” not an organization, during a Congressional hearing in 2020.

Trump has blamed the “radical left” for the killing of Kirk, who founded Turning Point USA, during a campus event in Utah. A 22-year-old man, who allegedly sent text messages admitting that he shot and killed the activist because of “hateful” views has been charged with murder in the case but his precise motivation has not been disclosed.

Other Trump administration officials have sought to curb negative comments about Kirk in the wake of his shooting. Vice President JD Vance encouraged Americans to call the employers of people criticizing Kirk. Attorney General Pam Bondi suggested she would prosecute hateful speech in the wake of Kirk’s death, before walking back those comments the next day. Defense Secretary Pete Hegseth has initiated suspension processes for service members who were viewed as making negative comments about Kirk.

Designating a domestic entity as a terrorist organization would mark an unprecedented step, potentially raising complicated legal questions. The 219 groups deemed terrorist organizations by the State Department are all foreign entities.