How did Santorum do in his last statewide election? Humiliated by 18 points. 2006 was a bad year for Republicans, but no other incumbent got slaughtered like that; Santorum’s loss was an order of magnitude more.
An 18 point loss in a “purple” state where all those Reagan Democrats live, Democrats we need to win a Presidential election.
So if Pennsylvanians got tired of his whinny, preachy demeanor, I don’t think he would wear well on the country generally.