A civil war in the United States won't be a thing. Unlike 1860's people aren't separated geographically along ideological lines. Second, the military wouldn't be involved and national guards would put down anything that would come close to instigating some form of nationwide violence. Third, states don't hold the same unity and stature they once had, with the exception of a few (Oklahoma, Texas, and Alabama arguably). So a 'civil war' would have no real hierarchy, planned military movements, long term campaigns, supply networks, or a long term goal with direction to that goal. Basically every aspect of a state wide war would be absent because the people who are capable of filling those voids will almost be entirely against the war. And even if some side could convince enough people to fight, who would be trusted as a leader in a rag tag army. There is just way too many variables that would almost have to be placed perfectly in line for this to occur from a grass roots standpoint.
Now, one feasible way of starting a civil war would be for democrats to take control of the house and impeach Trump for bogus reasons in a politically attempted cue. That would give two sides potential legitimacy depending on their views of the situation and put the American government and American values at stake. Those values are something that around 3 million men and women have sworn to protect and those men and women have the training, skills, and resources to fight because they are the American military. Likelihood? Unlikely. But of all the theories I've seen, that's the most realistic, and considering the vast majority of the Army is conservative, one can only assume who would win.