It is going to be very, very difficult for Mittens Romney to win the Republican nomination unless he is able to completely reverse the polling numbers that have just come out, and quickly. The caucuses and primaries that come up in the next month will form a sizeable chunk of the total delegate count needed to win (630 delegates up for grabs between now and 3/10). If Santorum maintains his momentum through March 10th, then the fact that many of the later contests (including California, with 169 delegates; New Jersey, with 50; Utah, with 40; Wisconsin, with 39; and Maryland, with 37) are winner-take-all, means that Romney's quest could spiral down the drain rather quickly.
The only way that Romney can win, in my opinion, is to hit Santorum hard. He is currently trying to do this, but the choices for his attack vectors seem particularly misguided. For the ultimate party insider (Romney) to try to characterize Santorum as a Washington insider probably won't stick very well. Similarly, hitting him on earmarks is a peripheral issue, at best, to the loony-tunes evangelicals that have already concluded that Romney isn't as strong as Santorum on their core social issues.
Ironically, the best attack vector is probably to hit Santorum where he may least expect it: on abortion.
Now, it really hasn't been well publicized, but back in 1996, Santorum's wife had a partial-birth abortion. While Romney doesn't have the cleanest (from the perspective of the wingnuts) record on suppressing women's control over their own bodies, publicizing this whole-hog, with a massive TV advertising campaign, will rapidly disillusion the support Santorum has developed amongst the Republican base. That won't necessarily mean that Romney will win that support, but he doesn't necessarily have to. All he needs is for it to be split between Gingrich and Santorum so that he can squeak through in the pole position in all of the winner-take-all states, and the nomination will be his.