Sanders, Schweitzer and O'Malley all sound like they have some willingness to run even knowing they'd lose. And each with their own reason.. Sanders wants to promote his issues and pull Hillary to the left, Schweitzer probably wants to set up a media career and O'Malley would be running for VP. (I bet he wouldn't attack her at all in a primary.) Because Schweitzer's wife is openly opposed to his running, I'd call Sanders and O'Malley 1-2.
Sanders seems somewhat reluctant. I mean if there was another good liberal with any chance at all running, there'd be no chance Sanders runs.