(I don't think all the stuff above is very likely, because I think the Trump hold on power is a lot shallower than most, but I do think there's going to be polling station violence in some places, continuing conflict between federal and state law enforcement, and a lot of lawsuits after the election contesting individual vote counts, far more so than Bush vs Gore. Especially over postal ballots.)
May I direct your attention to the Cultural Revolution? https://en.wikipedia.org/wiki/Cultural_Revolution
The possibility is there.
> The US military is completely unified behind the preservation of the US government.
I agree! But what happens if the question of "Who is the commander-in-chief" becomes unclear?
That is the possibility that's on the table. If there's an absolutely clear victory in the next election and the loser steps down, then we're in way better shape.
Posse Comitatus remains intact, and factional violence is left in the hands of domestic law enforcement (whatever that looks like at the time).
Which is what I'm seriously hoping for.
But if not... once again, I'm amazed to even be considering that possibility, but it's there.
That's splitting hairs.
Mao's Red Guards -- student revolutionaries and young adults -- effected a drastic shift in power, from a more liberal government to Mao (a strong authoritarian), and something on the order of three million people died as a direct consequence.
Moreover, China lost an unbelievable amount of its history, as revolutionaries purged the "Four Olds" (customs, culture, habits, and ideas).
Let me give you an examples of a Lebanese civil war
Second, it's not that US potential adversaries have ever cease attempts to recruit 5th column in the general staff.
A big double digit percentage of civil wars in 20th century were foreign power assisted, if not staged, and triggered by them.
US generals with extreme right political views are ripe for taking for a country that seemed to ace recruiting rightist elements for its cause. You know what country I am talking about.
It's the patchwork of law enforcement who are more willing to cause street-level trouble. We've already seen one protest which involved people bringing guns to a state legislature. What if there was another one, in response to an "abolish the police" candidate winning locally, and the police backed the occupiers over the legitimate state government?