Nate Silver points out that if Rick Santorum wins both Minnesota and Missouri -- where polls suggest he has a chance -- that "could revive his campaign, especially given that he also took Iowa."
"Mr. Santorum is, in many ways, a more dangerous opponent for Mr. Romney than Mr. Gingrich at this point. He has run a more disciplined campaign than the former House speaker, has less personal baggage and is less disliked by party leaders."
"Mr. Santorum can also make a credible claim to challenging Mr. Romney on electability. Mr. Santorum's current unfavorable rating among all voters is 11 points lower than Mr. Romney's, 36 percent versus 47 percent. Their favorable ratings are roughly equal: 30 percent for Mr. Santorum to 29 percent for Mr. Romney."