Never underestimate the Clinton machine. Bill is the one behind Hillary's campaign--he wants back in the White House very badly--and he is doing his usual calling to Dems up and down the line. Remember, most Dems want something from the government, and Bill is an expert at making very clear that unless they support Hillary early there will be no goodies for them when she wins.
The guys who supported Obama over Hillary in 2008 are being told that their prior treachery will be forgiven--if they get on board and get on board early.
Besides, the Dems know Bernie and think he is a kook (they say he is the Dems' Ron Paul), and they think O'Malley is a gaffe prone lightweight.
The view of many Dems is that while Hillary is a lousy candidate in many ways, so long as they can keep her away from ordinary people, do a lot of softly-lit commercials talking about grandchildren, the middle class and fairness (and hinting that all women should vote for another woman) she should be able to beat whichever Rep survives the coming Rep knife-fight, and then with her money they can do to the Rep what Obama did to Romney in 2012--define him as a bad person through massive ad buys making essentially false claims.
Hillary will be the Dem candidate unless she melts down like the Wicked Witch of the West. Which, of course, she is perfectly capable of doing.