The problem here is that even though liberal voters and donors might not like her there's no infighting among Democrats at the candidate level. Elizabeth Warren doesn't hate Hillary Clinton enough to run against her and bribing her to do so is against what makes Warren good.
More importantly, Obama is going to back the Clintons. He's still a rockstar in the black community. No matter how popular or unpopular he is next year with the national as a whole, if you don't have him to turn the black vote against Hillary, there's no way forward for a challenger in the primaries.
Probably stay neutral, but it seems highly unlikely he will. I kind of doubt that Biden wants his final act on the political stage to be getting curbstomped by Hillary. He'd rather be remembered as Obama's loyal VP and Senate dealmaker.
Any chance he comes back for Secretary of State under a Hillary Presidency? That's the job he really wanted.
