To quote Josh:
Sanders says clearly enough that Democrats are better than Republicans. But the core of his political message is that Democrats are fundamentally not on the side of working Americans and that their policies and politics for decades have been failures.
It’s been clear to me for some time that if Sanders were to win the Democratic nomination his general election campaign would be as much a campaign against the Democratic Party — its institutional structure, its past, its most celebrated leaders — as it would be of the GOP. Indeed, I don’t think many of his top supporters would even disagree with this. That is their diagnosis of American politics.
Emphasis mine.