Over at Plum Line, I go over the possible outcomes in South Carolina and what they would mean going forward. Bottom line is that it's over unless Santorum (or Perry) wins in South Carolina, or at least comes very close. And that's unlikely for Santorum, and basically implausible for Perry.
I didn't get to it there, but if Santorum wins in South Carolina, I'd say that Romney would then have a 75% chance of winning the nomination, maybe more. If Perry wins, I'd say it's closer to 50/50. However, the odds of Perry winning South Carolina or placing a close second appear to be long indeed; in fact, I think it's more likely that he looks at the numbers today or tomorrow and calls it a day than that he finds some way to revive things. So figure out the chances of Santorum beating Romney in South Carolina, multiply that by maybe 20, 25%, and that's your odds for stopping Romney's coronation. If this were a World Series, Romney is up three games to none, is up a couple runs early in game 4, and is the better team with a well-rested pitching staff ready to go.
By the way, I still don't understand why anyone thinks that Newt Gingrich is more dangerous to Romney than Santorum is.