I’m convinced that he never really wanted to be President and simply assumed that Hillary was going to be too tough to beat.
I think his goal was to be strong enough that Hillary would have to name him Vice-President. Then he could do what he’s good at: pontificating and voting present (except to break a tie in the Senate, but then Hillary would have told him how to vote). Further, he could spend most of his time out of the spot-light playing golf or taking vacations without anyone much caring.
Instead, he got thrust into a role he didn’t want and wasn’t ready for.