Are the Polls Missing Republican Voters?

Pollsters can take advantage of this if they use voter registration files, a large data set of every registered voter. Most private pollsters and a handful of high-quality public ones use voter registration files as the foundation of their polling, and these pollsters can use a respondent’s party registration to help ensure that their sample is representative. They can make sure that 37 percent of their respondents in Florida, for instance, are registered Democrats. In states where voters do not have the opportunity to register with a party, a pollster can use a voter’s primary participation history in a similar way.

If polls using partisan characteristics from voter registration files showed a fundamentally different race, this could be a sign that the other polls were biased on partisanship. But the recent surveys that are weighted by party registration or primary vote history offer nearly the same picture as polls that are not. Arguably, they offer a picture even worse for Republicans.

The Monmouth University poll of Pennsylvania this month, which showed Mr. Biden up by 13 points among registered voters, was arguably President Trump’s very worst poll of the cycle, considering the quality of the pollsters. The New York Times/Siena College polls last month showed Mr. Biden leading by nine points across the battleground states, and 14 points nationwide. A CBS/YouGov battleground tracker, which uses online polling data matched to voter file records, found Mr. Biden up nine points among likely voters nationwide.

Voter-file-based data lends credibility to polls showing a lopsided Democratic advantage in other ways. If all the various measures of partisanship — say, Republicans plus-two in Arizona or Democrats plus-two in Florida — are added together across the 94 percent of the nation with a measure of partisanship, Democrats outnumber Republicans by six points, 36 percent to 30 percent. This doesn’t prove that more voters identify as Democrats today, but the Democratic advantage in registration and recent primary participation nonetheless offers evidence consistent with the basic proposition that Democrats outnumber Republicans, and probably significantly.

Using this data, we found that self-identified Democrats outnumbered Republicans by eight percentage points in the Times/Siena national poll (the unrounded figures are Democrats 34.5 percent to 26.4 percent for Republicans). Among those characterized as Democrats based on party registration or primary vote history, 69 percent identified as Democrats in the poll; similarly, 65 percent of those characterized as Republicans identified as Republicans. Those without a party, disproportionately young and nonwhite, split toward Democrats by a narrow margin of 24 percent to 19 percent. At the same time, 6 percent of registered Democrats identified as Republicans, while just 3 percent of registered Republicans now identified as Democrats.

The irrelevance of partisanship

The polls that rely on voter files can help in another way: The pollster can tell whether there was a difference between Republicans and Democrats in their willingness to complete interviews. Pollsters using the voter files know the party registration of the intended respondent before they place the call. As a result, it’s easy to tell whether members of one party or another were more or less likely to respond to a survey.

If registered Democrats were likelier to respond than registered Republicans, it would undermine a core assumption of most public pollsters. It would also raise the possibility that bias contaminates the survey in unobservable ways: For instance, if Democrats were far more likely to respond than Republicans, one would have to assume that Democratic-leaning independents were also likelier to respond compared with Republican-leaning independents.