Martin O'Malley will run a respectful race in a democratic primary. If Hillary trips he'll run right by her. If he gets the nomination, watch out. I'm not sure the Country is ready for Rubio or Christie, the Republican Party is still a fractured mess and humpty dumpty won't be back together again.
O'Malley is too liberal for states like Missouri, Ohio, and even Pennsylvania. If he wins the nomination, and goes to the general, these are states that he needs to win desperately. I don't think that he can mount a significant challenge to a Chris Christie, a Marco Rubio or a Jon Huntsman. I think he would be a VP contender for Hillary or Elizabeth Warren (Massachusetts senator), but not the top of the ticket.
He's progressive enough to get the far-left vote, but in the general he will be trounced by Marco Rubio or definitely Chris Christie. He is probably too liberal (more liberal than Obama and Clintons combined) and he would probably pick a low-risk, low-reward vice presidential running mate who won't excite voters nor excite the Democratic base. He may lose.