The problem I see facing Bernie is that he thinks he's got more clout by waiting and I think he's losing that clout. If he had delivered the support when it was over (mathematically impossible after NY), he could take his support upstairs and get some actual concessions. After CA, he could've said "We're taking our message to the convention and back to the Party and going to continue the long-term battle, but right now it's time to get out there an stop Trump, so I'm supporting Hillary Clinton." He didn't, he chose to crap on her parade and make himself the overnight headlines. Similarly, his posturing after DC... He's not just figuratively but literally saying that he'll anoint her with support after she's proved that she's worthy of it.
I think that he's lost sight of the fact that with Trump trying to commit harikari every day, the pragmatist message is sinking in. "Yes, it would be swell to have the wise old sage of Burlington on the ticket, BUT HAVE YOU HEARD THAT ************ Trump? We've gotta stop him." More and more are shifting to Hillary; whether that's just "Not Trump" the votes still count. They're not going to vote "Not Trump" by throwing away their votes.
Are there going to be a recalcitrant number of Bernie supporters who won't vote for her? Yes. But they're not voting for her if Bernie throws his support behind her. The majority of the left who actually vote, a large majority, cannot sit idly by while a minority of the country elects a potential tinpot dictator. Trump's shenanigans with the judge and his ego-maniacal approach to Orlando may have cost him more votes than Bernie can deliver. And he's going to continue.
