Lynn Vavreck has an informative piece on the New York Times Campaign Stops blog today tracing shifts in presidential voting intentions from late 2011 through early September. The data are from the Cooperative Campaign Analysis Project, which interviewed nearly 44,000 people last December and has subsequently been reinterviewing 1,000 per week.

Through most of the spring and early summer, more than half of the survey respondents who were undecided last December were still declining to choose a candidate, with the rest breaking slightly for Mitt Romney over Barack Obama. Since around mid-June, more of these previously undecided voters have begun to commit, with Obama gaining and, in the last few weeks, surpassing Romney among those who were originally undecided. According to Vavreck, “These decisions seem largely to have been motivated by party identification.”

Meanwhile, both candidates have managed to retain the vast majority of prospective voters who supported them last December. Over the course of 2012, Obama has held 96% of those who supported him in 2011 and added 3% of those who originally said they would vote Republican. For his part, Romney has held 94% of those who intended to vote Republican and added 2% of those who intended to vote for Obama. (Vavreck notes that the 2008 CCAP study found almost as much stability in candidate preferences, with Obama holding 90% of his early supporters and John McCain holding 92% of his.)

To readers versed in election studies, these findings will seem very reminiscent of those from the first scholarly analysis of campaign effects: “conversion is, by far, the least frequent result and activation the second most frequent manifest effect of the campaign.” However, whereas Lazarsfeld and his colleagues in 1940 studied 600 prospective voters in Erie County, Ohio, Vavreck and her colleagues in 2012 have 44,000 nationwide. That’s real scientific progress.

[Cross-posted at The Monkey Cage]

Larry Bartels

Larry Bartels is a professor of political science at Vanderbilt University.