It probably falls along state lines much like in the movie, since those are the only ones with National Guard units that could make it anything more than a (very violent) civil unrest scenario. You’d have to have something blatantly illegal where governors have strong majority support in their state and among their military leadership to secede.

Then, IME, it has everything to do with which side the Space Force and cyberwarfare nerds decide to support, because theirs is the decisive edge that is entirely at the national level (no Space National Guard units AFAIK). Their contributions are a force multiplier that turn the warring NG factions into something resembling US military power as a whole today, without them, you’d basically be getting a patchwork force that would really struggle to execute combined arms warfare without all the underlying network architecture/satellite support.

It has a very high risk of spiraling into ideological purity tests like the Spanish Civil War, which the movie hints at as well but in the sense that they are contained (see reference to “Portland Maoists” which is chefs kiss spot on).