In smaller countries on other continents, we’ve seen radical changes in government since WWII up to current.
Could such a thing happen to the US of A of this century?
Is so, what type of platform of change and replacement government would be most likely?
I have no dog in this fight, just very curious.
If you believe Oliver Stone, we had a coup with the assassination of JFK.
If you believe some folks, we had a coup with Florida and SCOTUS in the 2000 presidential election.
And “stealth coups” of that kind are the only kind that could succeed in contemporary America. We just have not reached the level of broad disillusionment with the established political/social system that could make a mass-based revolution possible.
Yet. 
Damn but you lefties have big yaps for guys who ain’t even got the guns. 
Read It Can’t Happen Here by Sinclair Lewis.
(H. L. Mencken’s review of that book was titled “The Hell It Can’t!” Mencken was said to have slightly fascistic tendencies.)
And you know that how?