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.

Mr. Santorum’s conservative positions on social issues might not make him an ideal fit with certain types of independent voters. States that are moderate to -liberal on social policy, like Virginia, New Hampshire, Nevada and Colorado, could be tougher for Republicans to win if Mr. Santorum is their nominee…

With Mr. Santorum, however, you can at least draw up a coherent path to victory, one that runs through the Midwest. There is a Midwestern state left to vote at virtually every turn of the nomination calendar. After Michigan on Feb. 28 and Ohio on Super Tuesday comes Missouri (again) on March 17, when it holds its caucuses, then Illinois on March 20, Wisconsin on April 3 and Pennsylvania on April 24. (A big disadvantage for Mr. Santorum: He did not qualify for the ballot in Indiana, which votes on May 8.)