“but Trump is worse” is one approach. Another one that has backfired, especially among white voters, is the “prove you’re not racist by voting for me” argument.  They’ve been using it a long time, but it became especially evident in the Obama/Romney election when Biden told black audiences that Romney wanted “to put y’all back in chains”. The message was that only a racist could support a man that evil. I used to be a Democrat, but that was pretty much the moment in time where I realized I would never vote for a Democrat again.

I think a lot of people were tired of that kind of rhetoric, and it’s a major contributing factor to Trump’s victory in 2016. The Democrats didn’t take this as a lesson learned, and instead have doubled down on the rhetoric.  

You can’t insult voters and then expect them to vote for you. If Trump wins again, even now more than in 2016, that will be a key reason.