I have compiled the results of generic-ballot polls taken near to an election and compared them to the actual division of the Congressional vote. The table below presents the margin between support for the President’s party and support for the opposition. For each election I have used about half-a-dozen polls from different agencies taken just before voting day. Averaging the differences between these two quantities shows that these polls have fared rather well since 2002. The average deviation in the four midterm elections is 0.6 percent; in Presidential years that falls to 0.1 percent.
Still these averages hide some fairly wide fluctuations. In four of the eight elections the difference between the polls and the election results exceeds two percent. The error was especially egregious in 2006 when the polls predicted nearly a fourteen-point Democratic margin compared to 8.2 percent in the election itself.
In the most recent election, 2016, the polls predicted a slight positive swing in favor of the Democrats, but the outcome went slightly in the opposite direction. All the cases where the polls erred in picking the winner occurred in Presidential years and usually when the polling margin was close. The four polls taken during midterm years all predicted the correct winner, though the size of the victory was off by more than three points in two of those elections.