If not Obama, I think it'd be safe to assume either Clinton or Biden. However, I think a primary challenge to him would be disastrous.
It's too late for a primary challenge: the deadlines to get on the ballot in key states have passed or will in the next few weeks. Obama will coast to the Dem nomination, and if something does happen, Biden will take over as president, and will likely run for reelection. If, in that scenario, Biden decides to run for president, I doubt Clinton would challenge him in the general.
