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Most surveys conducted during the 2012 U.S. presidential campaign showed large swings in support for the Democratic and Republican candidates, especially before and after the first presidential debate. Using a combination of traditional cross-sectional surveys, a unique panel survey (in terms of scale, frequency, and source), and a high response rate panel, we find that daily sample composition varied more in response to campaign events than did vote intentions. Multilevel regression and post-stratification (MRP) is used to correct for this selection bias. Demographic post-stratification, similar to that used in most academic and media polls, is inadequate, but the addition of attitudinal variables (party identification, ideological self-placement, and past vote) appears to make selection ignorable in our data. We conclude that vote swings in 2012 were mostly sample artifacts and that real swings were quite small. While this account is at odds with most contemporaneous analyses, it better corresponds with our understanding of partisan polarization in modern American politics.
Gelman et al. (Mon,) studied this question.