As The Atlantic’s Molly Ball notes, there is a parallel between the endorsement Mr. Santorum received on Saturday and the backing he received from influential conservatives like Bob Vander Plaats in the days leading up to Iowa. Those endorsements helped to break a logjam between Mr. Santorum and other conservative candidates as voters behaved tactically and jumped on his bandwagon.

The extent to which there is a consensus of voters who want to defeat Mr. Romney in South Carolina is unclear. In the Public Policy Polling survey, 58 percent of voters said they would prefer another candidate to Mr. Romney. Nevertheless, Mr. Romney led hypothetical one-on-one matchups against several Republican opponents in the poll, including against Mr. Santorum, and Mr. Romney’s favorability ratings were reasonably strong.

The scenario in which Mr. Santorum would actually win South Carolina, therefore, probably requires two things. First, Saturday’s statement would have to get voters to take another look at him at the expense of other conservatives like Mr. Gingrich or Mr. Perry. And second, the negative attacks by Mr. Gingrich would have to succeed in softening up Mr. Romney’s support, but not play to Mr. Gingrich’s direct benefit either, allowing Mr. Santorum to leapfrog both and win with perhaps 25 percent of the vote.