Why Public Opinion Polls Don’t Include the Same Number of Republicans and Democrats

Newcomers to polling sometimes assume that if you are asking Americans questions about politics, it’s only fair to include an equal number of Republicans and Democrats. Although this notion makes some sense on the surface, it’s based on a misunderstanding of what polling is intended to do. The goal of a national political survey isn’t to artificially even the playing field. It’s to represent groups in their actual proportions within the country. And a wide range of evidence shows that there are more Democrats than Republicans in the United States today.

Gold-standard, nonpartisan surveys have found for decades that more U.S. adults identify with or lean toward the Democratic Party than the Republican Party—whether these surveys take place under GOP or Democratic presidential administrations. That is the finding of two of the highest-quality surveys that use nationally representative data collected through in-person interviews: the General Social Survey and the American National Election Studies. It’s also the result obtained by numerous other reputable surveys that poll Americans by telephone or online using randomly selected samples of adults, including those done by us here at the Pew Research Center, as well as those done by Gallup, Fox News, Kaiser Family Foundation and The Associated Press-NORC. (The Census Bureau, which runs the nation’s most authoritative surveys, notably does not ask Americans about their partisan affiliation.)

But polls aren’t the only source of evidence on this question.

In the 31 states that register voters by party, the number of Americans registered as Democrats outnumbered those registered as Republicans by nearly 12 million in mid-2018. Registration with a party is an imperfect measure of whether a person currently identifies with that party, especially in the South. But nationwide comparisons find that registration is closely associated with self-identification, and the two appear to be growing more correlated over time.

In addition, commercial voter files—which attempt to predict the partisanship of voters in all 50 states and the District of Columbia based on information such as address, race, age, and primary vote history—indicate that there are more Democratic registered voters than Republican registered voters in the country today.

Determining the right ratio

Of course, the important question for pollsters who want to accurately reflect the partisan composition of the country is not “Are there more Democrats than Republicans?” but rather “What is the correct ratio of Democrats to Republicans?” The answer to this question depends, in part, on whether we are talking about the general public, the smaller subset who are registered to vote, or the even smaller group of people who are likely to vote in a given election (“likely voters”). Evidence suggests that the Democratic advantage is somewhat narrower among registered voters than the general public—and often even narrower among likely or actual voters.

Among the general public, recent Pew Research Center telephone surveys find that Democrats and Democratic-leaning independents outnumber Republicans and Republican leaners by about 7 percentage points, similar to what the 2018 General Social Survey found. This Democratic-Republican balance is one of the factors we use to weight the Pew Research Center’s American Trends Panel (along with a core set of demographics including sex, race, age, region, and other characteristics measured in high-quality government surveys). All of our survey reporting is based on weighted data, rather than raw data that has not been adjusted to meet these parameters.

Among registered voters, the Democratic advantage in party affiliation is typically about 3 percentage points smaller than it is among the general public in our surveys. The reasons for this are well understood: Compared with the general public, registered voters tend to be older, wealthier, and more likely to be non-Hispanic whites and homeowners, all of which are characteristics associated with a higher probability of being a Republican.

Among likely voters—those registered voters deemed to have the greatest propensity to turn out in an election—or actual voters in recent elections, the share of Republicans is usually, but not always, higher than it is among registered voters, largely for the same reasons that registered voters are more likely to be Republican than those who are not registered. (Beyond the characteristics of the individual, a person’s likelihood of voting is determined by a variety of factors, including the specific candidates and circumstances in a given election.)

All of this is to say that there is no single “correct” ratio of Democrats to Republicans for all U.S. public opinion polls. And because the fortunes of the parties and their leaders ebb and flow over time, the ratio has tended to vary modestly over the years (albeit with Democrats representing a higher proportion than Republicans most of the time in the past couple of decades). Party affiliation is not a demographic characteristic like gender or race, which means people can change their affiliation based on what is going on in politics or because of other factors.

That said, party affiliation is a very stable attitude. This is also one of the reasons that different polls generally get similar—but not identical—estimates for party affiliation. Other reasons poll estimates might differ include sampling error, different weighting practices, variations in question wording and context, and differences in survey mode (that is, whether questions are asked by phone, online, or by other means).

Why election results might differ from the Democratic-Republican ratio in the general public

Obviously, just because more Americans identify as Democrats than Republicans doesn’t mean that Democrats always win the presidency (or control of Congress). There are many reasons that this is the case, including the fact that a sizable share of the public does not vote. Previous research has found that nonvoters are much more Democratic than are regular voters. Democrats also are somewhat more clustered geographically than Republicans, a factor that sometimes dilutes their electoral strength.

Candidates and campaigns, meanwhile, carry their own unique features that influence how or whether people vote. In 2010, for example, Republicans voted at higher rates than Democrats in the midterm election and won control of the House of Representatives, despite the Democrats holding a 3-point party affiliation advantage in Pew Research Center polls of registered voters that year. In contrast, Democratic engagement (and turnout) in 2018 was substantially higher than it had been in 2014, leading to a strong Democratic performance in the 2018 congressional elections and the Democrats winning control of the House.

So what does all this mean for consumers of political polling? In general, poll watchers today should expect to see more Democrats than Republicans in a national survey, particularly one designed to reflect the views of U.S. adults (as opposed to registered or “likely” voters). These partisan breakdowns don’t necessarily favor one side or the other; rather, they reflect the population in question as accurately as possible.

Courtney Kennedy is director of survey research and Scott Keeter is a senior survey advisor at the Pew Research Center.

This article was previously published on pewresearch.org and appears in this issue of Trust Magazine.