Democratic Primaries

by endorsing either of them.

Biden will likely be the nominee. He doesn't need any more endorsements. If he wishes to woo progressives in great numbers, he can always add Warren to the ticket as VP.

Sanders has not been kind to Warren during the course of the campaign. Why would she stick her neck out for him either?

If Biden wins, he will likely be a one-term president because I don't see voters being excited about reelecting someone who would be 82-86 during their second term. Whomever his VP is will likely have a large field to run against in 2024 just like in 2020, and I would expect that to include Warren (a likely front-runner), who will not want to do anything to damage her chances with progressives by endorsing Biden over Sanders unless she's getting something out of it (like the VP slot.)