What happened in South Carolina is really pretty simple. The Mormon Gap killed Mitt Romney. Defined as the percentage-point difference between the evangelical and the non-evangelical vote for a given Mormon candidate in a Republican primary, it turned out to be 16 points; i.e. Romney won 38 percent of non-evangelicals but only 22 percent of evangelicals. By contrast, Newt Gingrich won 44 percent of evangelicals, as opposed to only 33 percent of non-evangelicals.

In other words, had evangelicals voted like non-evangelicals, Romney would have won the primary, 38 percent to 33 percent. But since fully 65 percent of GOP primary voters counted themselves as evangelical, he lost, 28 percent to 40 percent. And lest anyone think that Gingrich, the Catholic convert, can't...