That poll was taken before the Romney PAC Opposition Destruction Machine has got going against Santorum. Romney is scorching the earth. He can't win, probably knows he can't win, and is making damned sure no other Republican can win. Romney doesn't particularly need to be president but he feels deep in his bones that he deserves the honor of being the Republican nominee and that the other candidates are trying to steal what is rightfully his.
If Santorum can handle the first two or three salvos with a combination of good humor and common sense, he’ll be well on the way to putting an end to both Romney’s Super Pac hate machine, and Romney’s candidacy.
But if Santorum can’t handle them, well, better to know it now than after he’s nominated.
Personally, I hope Santorum’s successful in parrying the inevitable attacks and that Romney finally gets his due for his consistent neglect of Reagan’s 11th Commandment. What goes around does come around...eventually.