The biggest reason Santorum is where he is right now is the negative ad blitz in Florida by the Romney machine. Otherwise Newt would’ve locked it up by now. No one wants Mitt or Paul, so up rose Rick.
Newt's mistake was getting down in the gutter with Mitt and getting off message. After watching those two flinging poo at each other many voters said, "A pox on both your houses!" The only other palatable contender was Santorum, so they took another look at him and obviously like what they saw. Newt could have weathered the Romney attacks if he'd stayed on message. Instead he looked angry and whiny.
There is another way to look at South Carolina and Florida, but no one seems to want to consider it. Santorum’s win in Iowa began to give him momentum. Then, before South Carolina, Palin and the other heavyweights came out with the “Vote for Newt in South Carolina, or Romney wins” meme. Without that, Santorum could have easily emerged then as the alternative to Romney. Instead, Gingrich arrogantly squandered the chance by taking Florida for granted. People are switching back to their first choice.