Trump has already said he planned to designate Antifa as domestic terrorists. Some are opposed to such a move, like this article from RAND.
One problem with that report; its author says at present the US only has a Foreign Terrorist Organisation list and "Antifa does not clearly meet the definition of the F, the T, or the O."
Since when?
Foreign: They are not American, it's an international movement following the same principles, waving the same flag and wearing the same identifying marks.
Terrorist: They've been using violence to silence political opposition for years. It is their entire raison d'etre. And if the past week hasn't show someone that they've escalated to killing people, that person's motivations are suspect.
Organization: These people don't suddenly appear in crowds through sheer happenstance. They coordinate activities online and have the same loose cell-based structure that all terrorist groups do.
The RAND article is a joke; hopefully someone in the Executive and/or Homeland will have more grit.
As far as I know RICO focuses specifically on racketeering (hence the name) and may need some evidence that the group is involved in criminal enterprises, not just crimes.
FTO or Domestic Terror designation just requires terrorist actions, which include incitement to political violence, especially when it is a threat to US national security and/or the economy, which (arguably) the actions of the rioters targeting federal facilities and police forces across the country for months, can be interpreted as. If a guy with an Antifa tattoo also killed a trump supporter purely because he was a Trump supporter, that can potentially be considered a political assassination.
I'm not for a second saying this would be an easy sell. Antifa, scumbags though they may be, are a very different type of threat to IS, or the cartels. However, they are a far more (in the USA) widespread and insidious threat and if they are allowed to push things incrementally further you may very well end up with a series of race-driven bloodbaths, which is precisely what these Marxist goons want.
I said before that the Black Militia (with shitty trigger discipline, what did they call themselves, the 'take no crap gang'?) could spark a major gunfight if they face off against an armed patriot group. All it would take is for Antifa to turn up posing as one side and fire off some shots. The longer this situation is left without a federal-level crackdown the more it will escalate.