"I will make it legal." -- Senator Palpatine

Sorry, in all seriousness though, I'm sure there's a legal pathway for any action the gov't wishes to take here [1].

[1] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2203713

Given its history, one should not assume the current administration will decide not to do something just because it's illegal or outside their traditional scope of power.

That said, I (not a lawyer) think it depends on how they do it. If they can compel Apple and Google to remove TikTok from the app stores (which I think they could with help from Congress), that would likely be enough to kill TikTok in the US.