The right (using the term strictly to define anyone who is right-of-center) is far too fractured to have some cohesive ideology that would be rallied behind. Moreover, there's no one geographical region that would be fought over - save for maybe the Capitol as well as the capitals of more than a few states. With or without a defined 'border' that would be fought over, what would most people even fight for? Sure, the Evangelicals could incite a domestic holy war. Yeah, there'd definitely be more than a few groups fighting to evict all non-whites. But generally speaking, I just can't see the masses picking up arms simply over some arbitrary group labels like "Republicans" and "Democrats".
The left? I think it's laughable to think that any significant number of average schmucks who identify as left-of-center would even want to wage a war against their fellow Americans. I'm firmly moderate and even though I vehemently oppose Trumpism and hardline social conservatism, I'd never say that I truly hated someone enough to pick up a gun and start shooting.
Worse yet, all the things that would be fought over would be destroyed in the fight. The economy, healthcare, social institutions - gone as we know them. And when they're out of the picture, what motivation does that even give someone to continue to rue his neighbor to the point of conflict? I think the biggest thing anyone would overlook in all this is that if our current infrastructure was demolished (literally or figuratively), a significant majority of people would starve, full stop. Our food supply chain almost exclusively relies on domestic and international peace and cooperation. Any severance of that chain (as would occur when other countries close their borders to the US) would shut down the nation's food supply.
All that's to say, Civil war - as in actual conflict - probably isn't going to happen. And if it does, then we're all fucked and unless you have a bunker + supplies, you're in it like the rest of us.
Autocratic rule? Definitely possible. I know the OP is not directly referencing this, but I like to envision this similar to the events of The Handmaid's Tale - maybe without the forced sexual servitude because as far as I can tell, the concern about the ability of women to reproduce isn't there. If anything, it's dudes that should be worried. So skipping that part, there'd be an insane number of refugees fleeing the country assuming we get to the point where our institutions of democracy have been usurped. And at that point, physical assets (guns, tools, machinery, gold, etc.) is what's going to put you ahead.
I would argue that the definition of the political far right is religious/ethnic tribalism. They can quickly coagulate around symbols and leaders (recall how Trump came out of nowhere, and within months was declared the savior). Right-wingers tend to quickly form organizations and fall into line behind leaders. That's why you see these storefront Pentecostal churches getting started within a matter of weeks and "militia" groups jumping from Reddit to real life within months. Right-wingers are collectivists who quickly respond to calls to organize. Old principles or religious traditions are frequently changed if there is an opportunity to create an attractive organization. It doesn't matter if they are "fractured"; today's right-wing is a diverse coalition of evangelical protestants, Catholics, anti-tax rich people, racists, gun hobbyists, etc. and never once have these fractures been an actual weakness to the movement.
On the other hand, the definition of the modern left-winger is extreme individualism. The general idea is "nobody can tell me what to do based on their supposedly higher status!" which is at odds with the concept of forming an organization. Organizations need leaders, a hierarchy, a method of attaining status and leadership roles, and rules/norms/restrictions for how people are allowed to interact with one another. Essentially, organizations create the inequalities and restrictions on personal liberty that offend left-wingers. So the modern left is more anti-organizational than anything. To the extent they support expanded government, it is because this is seen as the option to accomplish things without even more hierarchical and inequality-promoting private organizations.
The left wasn't always this extreme. Tightly-organized pro-equality organizations existed in the labor movement, civil rights movement, suffrage movement, abolitionism, and of course the American Revolution, among others. The past few decades have radicalized the left into an anti-organization attitude. Imagine left-wingers swearing oaths, wearing clothing symbolic of their group, attending meetings, and paying dues. That was the real world just a few generations ago.
Because organization = political power, the changing values on the left have led to a power vacuum, which is slowly being exploited by the right (slowly relative to the collapse of left-leaning organizations). The vacuum is steadily leading the US to being a one-party state by various means, and the Republican party has the opportunity to be more extreme and still win elections because they have layers upon layers of organizations full of passionate volunteers who are willing to take orders and follow rules.
It is the slowness of the transition that is frustrating right-wingers and leading to calls for violent overthrow. It is also the slowness of the transition that allows the left wing to live in denial about the inoperability of their value set, and to do nothing in the hopes that old government institutions run by political appointees will save the nation without much added input from them.
A left wing that is unwilling to pay the price associated with forming regularly meeting communities within a peaceful and prosperous society or unwilling to conform to anything less than their own individualistic preferences is not going to organize themselves into an army any time soon. They're not likely to join the police or military either. All these facts lead right wingers to ask "Why don't we just take it by force?"
Thus it is the weakness of the left which has led us to this moment of vulnerability for our two-party democracy. It is safe to say if left-wingers aren't willing to pay the much lower costs of organizing in today's peaceful and prosperous times, they aren't going to be willing to pay a higher price in a world of violence and deprivation.
To reverse the slide toward right-wing one-party rule, the left or middle-left would need to suddenly reject their cynicism about civic organization and go from being hyper-individualistic to being at least slightly collectivistic. I'm not sure how quick that can possibly happen, and I'm not sure if the left's insistence on absolute equality can coexist with left-wing organizations without creating an allergic reaction. How many people walk out when a rule is proposed about no outbursts during meetings?
The struggling left-leaning organizations I have observed tend to implode when their members (who are more pro-organization than those not attending) put their own interests, turf battles, and ideological pigeon holes ahead of the group's mission or the requirement to treat each other well (manners and social norms are conformism, not saying that ironically either).
I am carefully watching the younger generation to see if they show signs of rejecting the organizational cynicism and self-centeredness of their parents. That, in the end, and the amount of time remaining, will determine if two-party democracy continues in the U.S.
I was raised in a very conservative environment, and so was my wife. Ironically, just about everyone in my family and social circle who are conservative want one thing - to be left alone to be able to do what they want. Lots of "don't tread on me" and joking about the old saying about "I'm from the Government and I'm here to help". None, as far as I can reason, has any reason or will to actually take up arms. Fewer yet could even outline a basic thesis of government that would suit their needs other than something along the lines of returning to the Constitution as it was originally intended. Inevitably, what most of them really want is a veiled theocracy with "separation" of Church and state - with the caveat that Jews, Muslims, Hindus, Scientologists, Taoists, Buddhists, and all non-Christians GTFO.
I'm not trying to usurp your condensation of the motivations of each party, because I'd mostly agree with you, but from the environment I was raised in I still find it hard to believe that the "Right" has enough manpower (more than fringe groups) to properly seize power and maintain that power. The radical Evangelical/Fundamentalist seeds are there, but they as far as I can tell want to usher in the end-times to win the absolute favor of God while watching the secular heathens around them be consumed by the fires of hell. I do not jest but obviously not every theocratic-oriented organization holds their belief with that much fervor. But in any case, it's not a great way to run society.
Again, I'd like to point to how interconnected our society is with the rest of the world. Treaties, deals, trade agreements, pacts can be broken. We don't have the resources or manpower to Brexit from the rest of the world and I'm sure more than a few progressive societies would choose to cease doing business directly with the US. Without international cooperation and the image of democracy, our infrastructure would crumble, rapidly, and I have no idea whether would-be insurrectionists even realize that the Capitalistic wet-dream "leave me alone" society they pine for would not even be possible without the cooperation of more than the Republican minority.
Point being, without a long term goal, either/both a return to a Christian male-led Evangelical/fundamentalist or/and a free-market "Don't Tread On Me" privately run society isn't going to have much going for it for very long.
But I think your point about left-wing apathy is the wild card. I've shifted far away from the way I was raised - I just want to live my life in relative peace, free to do the things that interest me, without hatred of other people and with the intent of fostering that same environment for my kids and future generations. I know there are plenty of self-identified liberals who feel the same way, and when that worldview is threatened I hope the proper opposition to it would be generated.
ETA: I have been thinking on this a bit more and the real wild card is climate change. How little power man truly holds will be revealed against a shifting climate. What the fuck does it matter if you hate minorities when your electrical grid fails, or your town is demolished by a "freak" weather event. If we do end up with a single-party state, how little it will matter when we're all caught pantsless against forces so wildly beyond our control. If anything,
that should be the rally-cry for anyone who wishes to oppose those theocrats and xenophobes.
