Warren really wants to endorse Sanders and is waiting for Hillary to lose enough states for Sanders to be viable.
This. She needs to be sure that Sanders is a viable candidate. if he wins Nevada & pulls close in SC, I expect an endorsement before Super Tuesday.
The shocker will be Biden. As crazy as it sounds, I get the feeling nowadays than Biden hates her & if by end of April or so he is neck & neck, he will endorse Bernie. Ofcourse there is no reason for him to endorse him early, but a late endorsement is useless.
I think he will endorse late but at a time when his vote could still swing or cause some ruckus just to spite Clinton