Throughout the election season, the Pew Research Center and other major polling organizations report a measure that political insiders sometimes call “the generic ballot.” This measure is the percentage of voters in national surveys who say they intend to vote for either the Republican or the Democratic candidate for the U.S. House of Representatives in their district.* *(If the elections for U.S. Congress were being held today, would you vote for the Republican Party’s candidate or the Democratic Party’s candidate for Congress in your district?)

There is no national election for Congress, of course; rather, 435 individual races determine the composition of the House. So while it might seem that the generic ballot is too broad a measure to forecast the outcome, it has proven to be an accurate predictor of the partisan distribution of the national vote.

The final forecast of the generic House vote and the actual vote totals have paralleled each other very closely for nearly a half-century in U.S. elections. The average prediction error in off-year elections since 1954 has been 1.1%. The lines plotting the actual vote against the final poll-based forecast vote by Gallup and the Pew Research Center track almost perfectly over time.

For the most part, the generic ballot presents an accurate picture of the national political environment in mid-term elections. In 1994, for example, it showed the Republicans with a majority of the popular vote for the first time in 40 years, indicating that the GOP would make major gains. Republicans ended up gaining control of the House and have held the majority ever since. Since then, the two parties have been very close in the generic ballot, and the Republicans have clung to a fairly narrow advantage in the House.

With the House closely divided, the generic ballot is incapable of predicting which party will control the House if, as is currently the case, it finds voters evenly split between Democrats and Republicans. It also is less accurate during presidential elections than in the off-years. Typically, the generic House ballot question is asked after the presidential vote question, which may influence responses to the House item (See “Generic Congressional Measures Less Accurate in Presidential Years,” Sept. 18, 1996).

Yet in mid-term elections the generic is an important barometer of national trends. Going back to 1994, a survey conducted in July by the Pew Research Center showed the Republicans running about even with Democrats among likely voters — a reversal of historic patterns and an early signal that national conditions were favoring the GOP. In 1998, the final pre-election poll showed the Democrats drawing even with Republicans, which presaged the modest gains Democrats made that year. The generic ballot also is valuable for detailing the partisan preferences of major demographic and socioeconomic groups, and showing trends there as well.

So what does this mean for this November’s election? The generic measure has been virtually deadlocked all year. The most recent Pew Research Center survey of registered voters (conducted Sept. 5-10) shows a statistical dead heat with Democrats leading 46%-44%; when the sample is narrowed to likely voters the Republicans lead 47%-46%. In the five weeks remaining in the campaign, the generic ballot will be closely monitored for signs of a national trend, however slight, pushing either party into a clear advantage.