There is one thing I can be sure will happen next Tuesday: lots of polls will be wrong. Some will be
very wrong. Of course, we know better than to pin hopes on an individual poll; but as it turns out, the margins of polling averages are also more than a couple of points off most of the time. Considering the Daily Kos
Election Outlook has, as of this writing, 10 races within two points, we could have a wild ride this year.
Now here's the part you really wanted to hear: more often than not, the polls have underestimated Democratic performance. Don't get too excited though—there's a major exception, for conservative states. And that's where we happen to have some important Senate contests this year.
Here's how the polling averages have done in governor and Senate races, compared to actual results, for relatively close races between 2004 and 2013:
The diagonal line shows where, ideally, we would like to see the polling average: making a prediction that is exactly correct. The blue circles show where
Democrats did better than the polls predicted (above the diagonal line). The red circles show where
Republicans did better than the polls. Points above the red horizontal line are races that Democrats won; points to the right of the red vertical line are races that had Democrats ahead in the polling average. Points in the upper left and lower right quadrant show where the polling average incorrectly predicted the winner.
However, even when polls predict the correct winner they're off by a fair amount. Below, we'll look at some of the reasons why. We'll see that errors increase if there's not very many polls in the polling average or if there's third party candidates (and we have a lot of these this year). And, Democrats tend to outperform the polls more in blue states, while Republicans are more likely to outperform the polls in deep red states.
Please read below for a more detailed explanation.
