David Sirota's wholly right. Now that Bernie Sanders has scared Vt. Gov. Jim Douglas from the race and proven himself able to raise funds, there's no comprehensible reason for Democrats not to unite behind him. I recognize that he's an independent, but he's our independent, and as a lone wolf is able to tackle progressive issues and Republican misdeeds that our party, for reasons of legitimacy and comity, can't. That makes him a huge asset for Democrats who occasionally need a uber-progressive attack dog but don't have anyone willing to do it themselves.
If the party is concerned about the precedent of supporting an independent, they can simply pull out because he's "too strong to beat" and it's not worth wasting money trying to stop Bernie's juggernaut. That'll not only give them an exit strategy, but it'll also make Sanders look unstoppable, and help ensure no serious Republican dares waste his political capital on the seat.