Democrats need every Senate seat we can get, even a horrible right-winger like Joe Manchin. Manchin's only use is as a vote for Dem control of the Senate.
He would be useless as governor. Who cares about who the governor of West Virginia is? It's an insignificant state and it clearly wants Republicans to hold any office that isn't held by Joe Manchin.
I'm gotta make a bold prediction, he's gonna get Blanche Lincolned in 2018 if he tries to run for re-election in that senate seat.
It's only two years since he won with over 60% of the vote, and Obama will be out of office then. I think he's the favourite, although he'll very likely to sweat a real race.