in our soft existence we seem to have forgotten about massive domestic terrorism of the 60s and 70s orchestrated by leftist groups. there were bombings constantly, and everything was far less safe than now. any civil war would revolve around such tactics. after all, these are the kinds of things that have been shown to work against the american military
what could happen is that as all this gets more and more angsty from the left, they resort to terrorism again. unless quickly put down it can result in more and more, and people would choose sides. it would be a mistake, but a mistake that such groups are bound to make, to claim they are fighting "fascism" as it would just turn most people against them. now, if they instead were fighting FOR something that AGAINST something, they could get more mainstream support. but then again, they don't have to turn to violence to accomplish that.
being the opposition isn't working out for the democratic party, and they need to come up with bold vision that is easily communicated, and has broad appeal. since 2016 didn't make them learn, maybe 2018 will; it's not going to go how you want. run on calling for constitutional conventions. put forth your own bold ideas. say that if we win, we're going to remake the constitution for the 21st century. and explain what that means. changes to structure of government (eg, direct election of the president, for sure is one thing they'd want. i'd also say list-voting for house, while senate has direct-election, term limit for supreme court, etc). changes to the expressed goals of the government (make mention of national parks and the environment, of education and health care). changes to rights protected (i'm sure they'd argue against the second amendment, but the convention could come up with compromise, because it needs to be accepted by the american people, but could change stuff about internet, or about trannies or whatever, or aborting fetuses). either its done piecemeal or a giant remake, either way, we need changes to strengthen our institutions. the party in power isn't going to do shit about it as it just wants to consolidate power more.