Even if you don't mind Romneycare, or the abortion flip-flop, or any of the rest, there's a more basic problem: He's not a natural campaigner, and on the stump he instinctively recoils from any personal connection with the voters.
[To] the progressive mind, the very concept of "the enemy" is obsolescent: there are no enemies, just friends whose grievances we haven't yet accommodated.