President Trump declared late Wednesday that he was designating the antifa movement a terrorist organization, amid a broader effort by his administration to threaten liberal protesters and donors to progressive groups after the killing of the conservative activist Charlie Kirk.

Mr. Trump had made the same declaration in May 2020, but nothing came of it. And there are major factual and legal challenges to any government effort to formally designate antifa a terrorist group in any substantive way.

Here is a closer look.

What is antifa?

It is a diffuse and sometimes violent protest culture of far-left activists who want to stop the far right. The movement is associated with an aggressive form of protest its adherents call “direct action,” which can sometimes cross the line into illegal or violent activity like breaking store windows or setting police cars on fire.

“‘Antifa’ is short for antifascist and is most often used to reference activists and protesters who support more direct methods of confronting the far right,” said Jared Holt, a researcher of extremist movements at Open Measures, a company that monitors influence operations online. “Some who have self-labeled with the term have engaged in threatening or violent behaviors, but those individuals represent a small number of people who self-identify with the term.”

Is antifa actually an organization?

No.

Antifa is a label for a political subculture or protest style. The phenomenon does not have a leader, an initiation process, membership rolls, a headquarters, a bank account or a centralized structure.