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.
Violence recoils on the violent.
The stone often recoils on the head of the thrower.
No one cares to speak to an unwilling listener. An arrow never lodges in a stone: often it recoils upon the sender of it.
Crime oft recoils upon the author's head.
Every life is its own excuse for being, and to deny or refute the untrue things that are said of you is an error in judgment. All wrong recoils upon the doer, and the person who makes wrong statements about others is themselves to be pitied, not the person they vilify. It is better to be lied about than to lie. At the last no one can harm us but ourselves.
He for himself weaves woe who weaves for others woe, and evil counsel on the counselor recoils.