They thought she would win on her own, with but minimal assistance. By the time they saw that she would lose it was too late for them to step up the game.
Yes, although I don’t think anybody ever thought this nasty, lame candidate would win on her own. But the party got together and thought they were going to push her through and they had it nailed down - and then they didn’t.
The Dems did better this time, though, with an unpopular, lame, invisible candidnate...in fact two of them, because I think Kamala got almost 0 votes in her primary run - because they had it all set up in advance. They had to readjust because Trump was winning over the top. I’m hoping this will be their undoing.