It won't happen, at least not the way many fear/hope for. There won't be lines of people out shooting each other. There won't be state militia and guard against each other. There won't be patriots(sic) lining up to shoot or hang politicians.
If anything, it will be a cold war.
It will be a war of the media, a war of capital, and a war of information. Extremists on all sides will line up to boycott any business owned by someone of the opposite political cause- it doesn't matter if they are not active and will serve anyone- if they have a D or R behind their donations, they are the enemy to the opposite side. We will see the media even more polarized. We will see mobs of people demand that employers fire employees who think a certain way (how can you have such a racist hateful bigot representing your company?)
It will be a war of ideas.
BUT, I also believe anything like this will burn out quickly. There probably is 10% of people on each extreme who will actively take part in actions like this. They will be very vocal and seem like more than they are. But in the short time, the other 80% will burn out on all this very quickly. They will be made up of the uninformed, those who don't care, those who don't trust either extreme, or those who are just sick of it. They will be the ones who will look at their neighbor of the opposite political spectrum and decide that, no, they don't hate that person and they won't be led down that path.
I also don't believe there is one singular righteous cause (for lack of a better term) that will unify the masses. There are a lot of little events but they all galvanize around the game of politics.
Neither side is certainly not fighting for Liberty- the extremes on both sides just want control. Neither side is fighting for something like slavery. And no, I don't think Abortion is that issue even though people are passionate about it, those aren't the ones who would fight a 'hot war'. They certainly aren't going to take up arms because someone insulted their favorite politician.
At best, people won't leave the comfort of their recliners and what they think is the anonymity of the internet. Even the Tea Party burned out as it fractioned. Antifa even seems to have burned out, people got sick of them. You do see small little groups taking action, but those are 4-5 like we saw yelling at the Homeland Secretary director in a restaurant, not a massive movement.
I think we already passed some major issues people would have taken up arms over and no matter what they said was their red line, they just bitched and moaned on the internet. In short, the ones who are the most vocal are probably the most cowardly. They've been screaming 'CWII' for decades and never left their computers.