My guess is that Antifa, using demographic info provided by Democrat government workers, will show up early at polling place in heavy republican precincts, especially ones with older populations and cause a lot of trouble. Their hope will be that the information will be quickly spread through social media and will deter some from coming to the polls out of fear for their safety. It wouldn’t take too many such incidences in battleground states to flip the results from 2016. Trump won MI, WS, PA with razor thin margins.
That would be the kind of thing they would do. A few days ago they were circulating training materials suggesting that supporters should go to parking lots likely to be used by pro Trump people at a rally and fill up the parking spots.
No need for violence, just a traffic jam at or near the polling place.