Santorum failed to get re-elected, and has been very anti-McCain even though he was glad to have McCain’s help when he was running for re-election, so he has no chance of being chosen. I doubt a Congressman would be seen as ready to be President if something happened to the President. A conservative governor would probably make the best sense (that leaves out Huckabee).
Santorum's point was that McCain would make a bad President. And just tolerable as a Senator. The members of that club help each other with their fingers crossed. Like with a-holes in any profession.