Biden's delegate lead (over 300) has pretty much become insurmountable by Sanders. Biden needs to be free to focus on the General Election. Sanders needs to suspend his campaign, unite behind Biden, and try to convince his supporters to also unite behind Biden.
Current delegates per MSNBC:
Biden: 1,132
Sanders: 817
BIDEN CLOSES IN: Sweeps Bernie In 3 Big States
The faster they unite behind Biden, the faster they can bait and switch and go with Clinton. No way they are going with Joe. It will be quite a moment when Joe steps aside.
Just my opinion of course, but THAT would be something. Joe will have a difficult enough time getting Bernie voters who know they were shafted and many are now even questioning Bernie themselves. Many see him as just another weak candidate now, even if he campaigns for Joe. Some see the writing on the wall and questioned his inability to go hard at Joe.
Joe might get some, but Hillary or someone else wouldn't get any of their support. That would be the ultimate backhand to the flock who are already stinging.