Don't worry about the actual Democratic party taking any physical action. None of those guys have worked outside of an office for 30 years, they aren't taking to the streets with weapons now. The army won't take orders from them. The main threat from them is that they will try to drag the process through the courts for years. And then the endless indictments which they all believe will definitely, definitely work this time.

I also think the left has already shown the size of its strength on the streets. You won't get anything bigger than the biggest BLM riots so far. So plan for that size of trouble.

Then you just have to worry about individual terrorists or maybe new completely new tactics, maybe disruption of transport or utilities (which Extinction Rebellion did try in the UK).

Frankly the best prevention for this is just to calmly let Democrats you come across know Trump might win, why and that the world won't end. The main reason for crazy reactions is that people get stuck in a media bubble where they don't see the full range of possibilities then when something they don't expect happens they assume it's cheating, unfair or underhand. All that is needed for Trump to win is for more people to vote for him than the other geriatric rapist, no conspiracy necessary. That's what people need to hear.