Sanderistas pissed at Warren for not endorsing.

[Brittain33]

They're right. If she really cares about the issues she purports to care, she should have endorsed Bernie. The fact that she hasn't suggest she's giving more importance to her personal ambition.

Warren realizes Clinton is going to be the nominee and that endorsing Bernie would be throwing away her power without changing the outcome. Holding out preserves her leverage to get commitments from Clinton on her issues until an endorsement later in the general election season.