I think Biden may be trying to get an endorsement (and possibly more) from Warren to help get rid of the "Sanders problem"

I wrote a letter to her office directly. It can't hurt.

Dear Senator Warren,

I am not a constituent, I'll put that out there right now. But I recognize that you hold a very interesting position in the federal political landscape that governs us all. I wasn't going to write this letter, figuring being a non-constituent it would simply be thrown out, but the more I thought about it, the more I thought I had to. I too have a duty to our people, no matter how small it may be in comparison.

So today, Senator Warren, I need to ask a favor as concerned citizen, as a fellow woman, and as a human being inhabiting our beautiful planet Earth. I am writing to you today to ask you to please endorse Senator Sanders' campaign for president. Over the years there have been only two consistent voices that I could trust in politics, and those two voices belong to you and him. You both speak in plain English, in common sense, and with an obvious sense of heartfelt outrage at the complete destruction of America being carried out from within by the monied interests.

Senator Warren, I'm begging you to help us change America for the better. Help us get the money out of politics. Help us break the establishment gridlock. And help us leave a better world, and a better system, for our children.

Your public support of Senator Sanders would be one massive step toward a return to freedom, opportunity, and a true and honest hope for the future outside the bounds of establishment. I urge you to be bold, for all our sakes.

And I thank you for your time.

Regards, Sniggletypuss

/r/SandersForPresident Thread