I'm with
@SpanishSpy that a very significant opposition force - after the usual suspects of foreign exiles, the churches & conservative groups, and discontented intellectuals & unions that lost out (even if this is a very nice communist state running on trade unions
someone will be unhappy or lose out) - is going to be racial, because the communist state is going to have to pick a side there, either they're backing black workers or backing the
legitimate concerns of certain white people. Undocumented immigrants might also be an issue, are they fellow workers to be accepted now they're there or are they foreigners? And do the Native Americans get left alone or are they told "guess what you're communists now too"?
A lot of that will be determined by what groups participated in the revolution and the coalition building that happened to make it win. On race, I feel like white supremacy is just too useful to the reactionaries. If it's early enough, whiteness as a construct isn't even going to be what we know now. A lot of immigrant communities were excluded from it initially and may rally to a radical movement because of that. Black workers are never going to fit easy in the pre revolution system so it's likely they'll be reached out to. Which isn't to say all of the unions and organizations that also join in would be positive on the move, but it's a pool of people with grievances I can't see a revolutionary build up ignoring.
As for intellectuals, you'll probably get plenty who aren't against it either. Whether that's artists, students or victims of the reactionaries' own coalition building, which is likely to embrace the worst of the American right in search for fighters.
Actually, one adjacent problem the revolution would have more trouble with than academics is liberal professions. They'll probably have to give some concessions to keep doctors and the like around and integrate them into their system. Not that it's impossible, Britain managed to keep them on board despite the NHS being radical change for the profession. But definitely something to keep in mind.
On churches, a question could be what role more radical theology plays into conciliating religious people with the revolution. America has always had a streak of Christian socialism, and while Marxist orthodoxy is of course not invested in that as a solution, I can see that being part of any successful revolutionary coalition. This doesn't magically erase issues with churches even if it open paths to convincing believers, of course, and I still expect the more conservative churches to serve as rallying points for reaction, both during and after the revolution. The fact America doesn't have one hegemonic church organization like Russia or Spain means the revolution is more likely to be selective on what churches it opposes though, and how much opposing is needed will depend on areas. I'm especially thinking about white churches in the south, which would be a problem.
It'd be interesting if they tried to force some of the Indians back into "primitive communism" or something.
Why though, beyond projecting reflexive anti communism on the situation.
A big debate for the reservations and native populations in general will be the existing splits between community leadership and their population, since those don't always align. There's also the question of the communities that didn't have reservations before, and all those who were excluded by blood quanta laws. A common complaint is the inability to continue adoption practices communities had before being forced into the settlers' framework, and it's easy to see a communist revolution doing a full rewrite of the law, at which point they probably wouldn't reintroduce the same framework to police native identity.