Following 2018, all the Dems had to do was play it cool, work with Trump on some issues, avoid scams like Mueller and Impeachment, and run a moderate in 2020. Worst case, Trump wins a second term, but Dems retain house and likely get a senate majority. Best case, they win all.
But, they couldn't contain themselves. Instead they got drunk with power in the House and in the media, and doomed themselves to a blow out loss in 2020 and total rebuild of the party.
The Dems really have no one to blame but themselves at this point. With nothing to lose, they will dunk Sanders and go with Clinton at the convention. I have no doubt about that because they will need to protect down ticket races in any way they can.