People forget too quickly that Obama was the longshot candidate in 2008. He was the less than one term senator from Illinois whose biggest prior accomplishment was speaking at the democratic convention in 2004 (also being the former president of Harvard Law). At least Bernie is a legit candidate with a solid background. Unless it comes out that he's been hiding a kid that he's denied child support or that he's not actually a secular jew, but a satanist with voodoo tendencies, then there is no real way to attack him other than calling him a socialist which he doesn't even deny being. And that didn't really work against Obama.

"Why Bernie Sanders is Actually the Democrats' Best General Election Candidate". Pretty much says it all, for the Dems.