Now he can do what they did in the 60's and he's already doing it. He's already calling people names and belittling people and intimidating anyone who does not agree with him. (Look at ABC and Jimmy Kimmel)
He can designate certain groups as "gangs", if they engage in frequent criminal misconduct, such as the Elm Fork John Brown Gun Club, but of course it doesn't sound as cool as "terrorist".
Elm Fork is an antifascist gun toting group who had some members attack an ICE facility back in July.
Gifted article from WaPo
What the law says as Trump tries to designate antifa a terrorist group
The U.S. has no legal way to designate domestic terrorist groups. Experts raised fears of political repression because antifa is a broad ideology, not a group.
Can Trump designate antifa a terrorist organization?
There is no legal mechanism in the United States for labeling purely domestic organizations as terrorist groups. For Trump to follow through on his social media declaration, his administration would need to create such a mechanism. The Trump administration has shown a willingness to push legal boundaries in ways that many experts hadnt expected, and such an effort would surely draw challenges, given the Constitutions broad First Amendment protections for free association and speech.
The president does not have legal authority to designate a domestic group as terrorists for good reason, as any such designation will raise significant First Amendment, due process and equal protection concerns, said Hina Shamsi, director of the American Civil Liberties Unions National Security Project.
Trump appears to be employing a term that many people associate with left-wing violence to try to promote a repressive agenda, Bray said, adding that such a move could provide an umbrella for clamping down on any political opposition.
Under U.S. law, the government can designate hostile groups as foreign terrorist organizations, a label that carries key financial and legal consequences. It can do so because individuals and groups that are entirely foreign arent protected by the First Amendment. Typically, the formal declaration is made by the secretary of state, often under direction from the president, and the label allows the Treasury Department to impose financial sanctions and the Justice Department to prosecute people for providing material support or resources to the organization.
https://wapo.st/4mjuSIT