What seems to have become clear is that the hypothetical candidate these voters might have preferred to Mr. Romney has not materialized.

As Lynn Vavreck and John Sides, a FiveThirtyEight contributor, note, Mr. Romney has considerable leads over candidates like Mr. Gingrich and Rick Santorum when tested in hypothetical one-on-one match-ups.

The same thing holds in South Carolina. In a recent Public Policy Polling survey there, 58 percent of voters said they’d theoretically prefer another candidate to Mr. Romney. However, no actual Republican got more than 39 of the vote when tested against Mr. Romney in a one-on-one match-up.

There are enough substantive and stylistic differences between the various “not Romney” candidates that they should not be viewed as interchangeable, this evidence suggests. A considerable number of Rick Santorum’s voters prefer Mr. Romney to Mr. Gingrich; a considerable number of Mr. Gingrich’s voters prefer Mr. Romney to Mr. Santorum.