For that reason, if Mr. Santorum and Mr. Romney look as though they’d do about as well the other in the Midwest, much of Mr. Romney’s electability advantage dissipates. And a candidate who gets 49 percent of the vote nationally but 51 percent in the Midwest would arguably be better than one for whom the reverse is true.

Apart from the Electoral College, there are some other “candidate quality” factors that work in favor of Mr. Santorum. For instance, his favorability ratings are currently better than Mr. Romney’s among general election voters. That could change — Mr. Santorum has not been faced the same scrutiny from the media or faced the attacks from opponents that Mr. Romney has, and favorability ratings can revert to the mean. But it’s harder for Mr. Romney to make a case on electability when his favorability ratings are net-negative with the electorate as a whole and with independent voters specifically.

Also, Mr. Santorum might do more to motivate the Republican base than Mr. Romney. The difference isn’t likely to be large, but if we’re looking at things that might make a marginal difference of 1 or 2 points, it is worth thinking about.