Explanation of the generic ballot and relationship to predicting midterms:
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.