That being said the DNC did scrutinize him pretty thoroughly during the primaries, true they didn't go straight for the jugular but again I'm not sure it would have worked. Do you think all the stadiums of people who chanted his name would so easily believe attack ads?
Yeah, no he wasn't. They didn't go for anything, jugular or otherwise. The Clinton campaign didn't attack Sanders, they ignored him because he was a nonentity that could never win. The smart thing to do, in retrospect, would've been to go scorched Earth and burn him to the ground.
As to the stadiums of people you reference, they like the image of Sanders. He is a Twitter candidate, appealing to the lowest kind of voter (intellectually) who like him because social media told them to. But the thing with those voters is, they are very easy to sway because they hold no strong convictions or opinions of their own. As to how they would destroy him, I will just copy and paste another post I've made as to the opposition research that exists on Bernie Sanders:
"How likeable do you think Sanders will be when his essay about women secretly enjoying/wanting to be raped is released? How about when it's widespread news that he spent his entire adult life on welfare, literally right until he got elected? He has literally made a living off of sucking the teet of the taxpayers. How do you think that plays? Or during his time on welfare, literally stealing electricity from his neighbors and forcing them to unknowingly pay increased bills while he benefited? So likeable. And let's not forget Sanders actually going to a rally for Hugo Chavez, a dictator, mass murderer, and tyrant, and calling him a great patriot. Those videos will really play well. Especially the parts where he embraces the dictator, mass murderer, and tyrant. He will have the Latino vote locked up with that, I'm sure.
There is more, but none of this came out because Sanders was never realistically going to win. Clinton wasn't going to hit hard out of fear of alienating his supporters, not when he wasn't ever a real threat. And Republicans wanted to prop him up to advance the "Clinton screwed Sanders" narrative. But in a competitive primary or a general election, it all comes out --- and it would sink Sanders over night."
Sanders is not a strong candidate. He is a loud candidate who is very good at drawing attention to himself, but he won't survive the scrutiny of a real a presidential campaign. Clinton never pushed him because there was, at the time, no benefit to her in doing so. All it would do is further divide the Party. But if he tries to run again, all of this comes out.