One possibility is that the voters who said they were undecided broke decisively in Santorum’s direction over the last few days of the campaign, but the data do not provide much support for that theory, especially in Alabama. According to network exit polls in Alabama, Santorum did just as well among those who made up their minds in the last few days as among those who decided earlier: 32 percent in each category favored Santorum. In Mississippi, Santorum ran eight points better among late deciders (40 vs. 32 percent), but still ran even with second-place finisher Gingrich among those who said they decided sooner. The three Mississippi pre-election polls had found Santorum running an average of 5 points behind Gingrich.

A second and more popular theory is that some Santorum voters were “lying” or otherwise reluctant to admit to a live interviewer that they planned to vote for Santorum. Prior research shows survey respondents tend to underreport “sensitive” behaviors to a live interviewer (such as smoking, drinking or sexual activities) that might create “social discomfort” in the conversation. So the theory goes, admitting to support of Santorum might involve a similar sense of embarrassment. Note, however, that the automated polls also understated Santorum’s support in these two states, just not as much. Moreover, it is unclear why such an embarrassment factor would be significantly greater in Alabama and Mississippi than in other states that have already voted.

A third and more compelling argument is that the polls tended to sample likely Republican voters too broadly, thus under-sampling constituencies like evangelical Christians or strong conservatives who are more supportive of Santorum. Comparisons are difficult given spotty disclosure about the pre-election polls, but on this point the exit polls provide some support. For example, PPP found that 68 percent of its sample in Alabama and 70 percent in Mississippi self-identified as evangelical Christian. In the exit polls, 80 percent in Alabama and 83 percent in Mississippi identified as born-again or evangelical Christian (although note that the wording of the question was not identical — the PPP question did not include the descriptor “born-again”).