What Would It Mean for President Trump to Declare Antifa a "Domestic Terrorist Organization"?
Not much as a legal matter, as best I can tell. [See UPDATE.]
A week ago, President Trump said:
The United States of America will be designating ANTIFA as a Terrorist Organization.
— Donald J. Trump (@realDonaldTrump) May 31, 2020
It's not clear to me, though, what (if anything) this substantively means. The Secretary of State can designate foreign groups "Foreign Terrorist Organizations," which makes it a crime for people to give them various "material support or resources" (see, e.g., Holder v. Humanitarian Law Project (2010))—which includes giving them money and other tangible goods, giving them your services (by joining them and doing work under their control), giving them certain kinds of advice, and the like. But I don't know of any law that would similarly cover domestic groups, such as Antifa. (There are of course people in other countries that call themselves "Antifa," and doubtless there is some communication among American and foreign people who labeled themselves this way, but I can't see how that would be enough to trigger the statute, especially given the First Amendment.)
Now of course there is already a well-established legal category for domestic terrorist or otherwise criminal organization: conspiracies. If people are conspiring to burn or bomb or even just vandalize abortion clinics, or animal research facilities, or synagogues, or buildings that happen to be near demonstrations, or for that matter the property of a rival mafia family or street gang, that's of course a crime. And there are particular crimes defined as "domestic terrorism"—"activities that":
(A) involve acts dangerous to human life that are a violation of the criminal laws of the United States or of any State;
(B) appear to be intended–
(i) to intimidate or coerce a civilian population;
(ii) to influence the policy of a government by intimidation or coercion; or
(iii) to affect the conduct of a government by mass destruction, assassination, or kidnapping; and
(C) occur primarily within the territorial jurisdiction of the United States.
When people conspire to commit such crimes, they may indeed be a sort of domestic terrorist organization (or, if you prefer, a conspiracy to commit domestic terrorism). Abstractly advocating or praising such behavior isn't a crime, and is indeed protected by the First Amendment. Indeed, joining an ideological group that has some unlawful ends is constitutionally protected unless it's done with the "specific intent of assisting in achieving the unlawful ends of the organization." But engaging in various kinds of violence is a crime—and is constitutionally unprotected—as is conspiring with others to do so (which is to say, generally speaking, deliberately agreeing to work with them in their criminal projects).
Still, there's no legal significance in labeling a particular conspiracy or set of conspiracies a "domestic terrorist organization," or in using that label for a political movement. Perhaps it might send a signal to federal investigators and prosecutors to look at the organization more closely—as they would generally be free to do, even as to ideological groups, when there's serious reason to think that those groups are actively planning criminal conduct. I certainly hope that the FBI and DoJ have closely investigated all sorts of violent ideological groups like that, and are continuing to do so for any other such groups that continue to be threats, whether they are white supremacists or radical leftists or ecoterrorists or violent anti-abortion groups. But, as best I can tell, all the government can do with regard to those domestic groups (again, unlike designated foreign terrorist organizations) is investigate them and prosecute their members for actual conspiracies to commit specific criminal acts.
UPDATE: Kevin Gerson, Director of the UCLA Law Library, notes that domestic terrorist organization status does matter in one context: 8 U.S.C. § 1182(a)(3)(B)(vi)(II), which provides that alien members of designated terrorist organizations—foreign or domestic—are not to be allowed into the U.S. So President Trump's planned designation might mean that alien members of domestic Antifa who are trying to come to the U.S. won't be allowed to enter, and may even be deported. I doubt that this would have much effect on Antifa, and I doubt that this is what most voters would expect from President Trump's statement, but there it is.