What you see, on this as on multiple issues, is the casual adoption, with no visible effort to check the premises, of a story line that sounds good. It's all about the big banks; single-payer is there for the taking if only we want it; government spending will yield huge payoffs — not the more modest payoffs conventional Keynesian analysis suggests; Republican support will vanish if we take on corporate media.
In each case the story runs into big trouble if you do a bit of homework; if not completely wrong, it needs a lot of qualification. But the all-purpose response to anyone who raises questions is that she or he is a member of the establishment, personally corrupt, etc.. Ad hominem attacks aren't a final line of defense, they're argument #1.
I know some people think that I'm obsessing over trivial policy details, but they're missing the point. It's about an attitude, the sense that righteousness excuses you from the need for hard thinking and that any questioning of the righteous is treason to the cause. When you see Sanders supporters going over the top about "corporate whores" and such, you're not seeing a mysterious intrusion of bad behavior into an idealistic movement; you're seeing the intolerance that was always just under the surface of the movement, right from the start.
Does Clinton have problems too? Of course — she's been too cozy with established interests in the past, she shouldn't have given those speeches, and of course she shouldn't have voted for the Iraq War. But there is no evidence that she's corrupt, and lots of evidence that she both thinks hard about issues and is willing to revise her views in the light of facts and experience. Those are important virtues — important *progressive* virtues — that seem woefully absent on the other side of the primary.