They cheered for Sanders when he said, "We have got to defeat Donald Trump!", and then a second later booed Sanders when he said, "We have got to elect Hillary Clinton!" The booing continued as Sanders reminded his booing supporters that, "This is the real world we live in!" Yes, it is. And here in the real world the only way to defeat Donald Trump is by electing Hillary Clinton. There is no alternative — Bernie isn't going to get the nomination, Jill "Plan B" Stein isn't going to prevent Donald Trump from implanting (or whatever), and Gary Johnson isn't going to win the election.
Listen to Bernie, Berners: Want to defeat Trump? Elect Clinton. Period. The end.
Remember when progressives liked to say they were proud members of the reality-based community?