Republicans would see through this. Also, Cruz is not the least electable candidate, IMO. His intelligence may be vastly overrated by this forum, but his electability is a bit underrated by the media. I think he'd lose, but Democrats should hope for someone like Carson to win the nomination, as he would likely get destroyed in a GE, in a way that Cruz probably wouldn't.
I don't see why they wouldn't just want Trump to get the nomination. Carson says some crazy things but has a certain likability factor that Trump couldn't buy with all his money.