If I absolutely had to pick somewhere, I would guess the West coast / Pacific northwest. Yes, they have some of the loudest pseudo left capitalist lib cringe shit, but they also have a big socdem/demsoc population (Bernie people ripe for radicalization, at least moreso than most other types of Americans) and a not-nonexistent communist population. It would be feasible to pull off as they have a HUGE economy, manufacturing, tech etc. so independence from America wouldn't fail overnight. They also have a large population of fascists that would be a huge thorn in their side--the "state" of Jefferson could bleed California dry (literally, lots of California water passes through their territory) before they had a chance to retaliate.

This is just going off of what I know, and now. There's also several major cities that have huge potential and I could see an actually-based actually-effective pragmatic CHAZ type deal developing in one, even if it didn't last (all depends too on how powerful the American federal state is at said point).

The more interesting variable I am looking towards is our lumpenproles. We have written them off as all irredeemably fascist and certainly terminally anticommunist but I don't see this as necessarily set in stone, it would be amazing if we could reach the disaffected rural Americans with an explicitly communist program and this would flip the entire situation on its head. I have lots of experience talking to "salt-of-the-earth" Americans and many of them are more confused and misled than dedicated to a self replicating ideology of fascism or capitalism or anti communism. I have found many will even espouse communist and egalitarian talking points (stripped of buzzwords, of course) with little rhyme or rhythm--in between capitalist, fascist, and bigoted talking points of various degrees. There is definitely a hunger for something they don't realize that only communism can provide, a hunger for meritocracy and rationale decision making and an end to corruption and exploitation. They are bitter and resigned to "the way things are" in many ways and trillions of dollars of Red Scare has disfigured their perspective, but man oh man, if that levee were to break even for a second...America would wash red in months. Both in a good way and a bad way. I definitely don't think our ruling class would relinquish without a nasty fight, not that I don't wish to be wrong.

I'm really talking out of my ass here but some of this is based on harder evidence, e.g. Google Bernie donations you will find a huge chunk in California--it's not an end-all be-all but what am I going to do, find a heat map of genuine communists in America? Good luck...

I also want to see how foreign interference might work. If USA got weakened enough, would China drop its policy of playing nice with the capitalist world hegemony and try and push some resources towards a revolution in America? Or something similar? Would the capitalists flee, if so where? Could they establish a new, desperate, last attempt of an America on some other lands?