They have at least two other options: vote third party or don't vote at all. There will be some of each among GOP voters, more than usual, I suspect. Will it be enough? We'll see, but I hope so.
I hope so too but I think it would be foolish of the Democratic Party to rely upon it for electoral success. Instead they need to find a candidate who is positively enticing, a Bill Clinton or an Obama. Perhaps someone who has learned from Trump to "own" past indiscretions rather than continually apologise for them and allow the media to bully them.