 They thought that that was their best chance against Trump. And whatever you think about the logic of that or how much that really makes sense to a sane person, they were able to pull that off for a variety of reasons, one of which they stirred up so much hatred for Trump. Another reason is that they have a voter base, a portion of their voter base that has no idea what's going on. They would vote for a thumbtack if the Democrat Party and the media and Hollywood told them to do that.