However, it’s just as plausible that voters who chose Mr. McCain for other reasons (such as party loyalty) were echoing one of his campaign’s primary themes back at pollsters. Why would McCain voters choose “bring[ing] about needed change” as the most important candidate trait to them? Change was one of Barack Obama’s main themes in that election; choosing it would make little sense. (Not surprisingly, Mr. Obama dominated among those who chose “change” as the most important trait.) In cases like this, exit polls can’t distinguish between the factors that determine people’s votes and rationales they construct after the fact.

Another problem is that people’s reports about their intentions or motivations may not correspond to the factors that have the most influence on the outcome, especially in the most important districts or states. In 2010, for example, Democrats actually won the House vote among exit poll respondents who said health care was the most important issue to them — a finding that might seem to suggest that the issue helped the party.

However, research I conducted with the political scientists Eric McGhee, John Sides, Seth Masket and Steven Greene found that votes for the Affordable Care Act were uniquely damaging to the electoral fortunes of Democrats in 2010 and may have cost them control of the House. Its effects appeared to be concentrated among members in competitive districts – precisely the legislators who were most vulnerable to defeat.

Finally, people may simply not be honest about their actions or aware of the factors that influence them. For instance, one analyst concluded that “race was not a decisive factor” in the 2008 election because Mr. Obama led Mr. McCain by a similar margin among those exit poll respondents who said race was an important factor in their vote and those who said it was not. In fact, the best estimate (constructed from anonymized Google search data by Seth Stephens-Davidowitz, a Times contributor) suggests that racial animus cost Mr. Obama approximately four percentage points of the national vote in 2008 (and in 2012 as well).