I think he might run. And in any case....
He will be able to win in SC and will eventually edge out over Gingrich ("serious issues") in the South. The Convention will be deadlocked but will decide to pick Southern conservative DeMint over Northeastern moderate Romney. He will choose Jan Brewer as his running-mate. The map will look like this:
Senator Jim DeMint (R-SC)/Governor Jan Brewer (R-AZ) 51.3%, 274President Barack Obama (D-IL)/Vice-President Joe Biden (D-DE) 49.2%, 264Nevada is the closest state. It sides with the GOP mainly due to the economy.
Of course, I'm not saying it'll happen. This is just what it would be like if it DID happen.