Bernie, when endorsing Hillary Clinton, didn't combat the aggressive rhetoric from his toxic base that voting third party/Trump would hand the election to Trump. I think he did poorly communicating to his supporters about the progressivism of Clinton. There's no reason why so many dopey sore losers would turn to Trump when the records were set clear.
"The progressivism of Clinton"? What are you smoking?
The proper thing to say - which he kind of did but not loudly enough - was "there are no progressive options left to vote for. We must therefore vote to prevent Trump and/or vote for the lesser of the evils to get as close to progressive policies as possible out of the remaining choices. Either way you approach it, Clinton is the answer."