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:
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.
As mentioned above, we have a huge number of races with third party candidates this year, and they don't seem to be fading away as election day approaches. In the past, this has led to large errors in the polling averages—two-thirds of the time, more than four points in the margin. Here is what that looks like for the closest races:
Another source of error in polling averages is too few polls. This isn't much of an issue in close races, which are usually polled frequently, unless the race is changing faster than the pollsters are polling. Below, the yellow circles have 10 or more polls in the average, while the green circles have fewer than five.
Once we narrow down the data set to just those polling averages with five or more polls, and no races with third party candidates getting more than 5 percent, we can find a familiar pattern based on the partisan lean of the state showing up.
Below, the blue circles show only the elections in states where President Obama won more than 55 percent of the vote in 2008:
Below, the mirror image: elections in states where Obama won less than 45 percent of the vote in 2008:
As you can see from the graph, these races are more likely than not to have polls that are too friendly toward the Democratic candidates. On average, however, it is only a one-point error. Ancestrally Democratic states such as Arkansas, Kentucky, and Louisiana might not consistently show this effect, however.
A practical guide:
Here's a summary of what the last ten years of polling errors can tell us.
1. Look at the number of polls you have. If you have only a handful of polls, your polling average will likely have much greater error.
2. Look for third party candidates. If you have a third party candidate polling around 6 percent or so, the polling average is likely to be off by five or more points on the margin. This is true for many races this year.
3. If you have plenty of polls and no or low-polling third party candidates, look at the state. Deep blue states will, on average, have polls that are too Republican by a few points, while deep red states, on average, will have polls that are too Democratic by a point.
4. Finally, remember that past performance is no guarantee of future results.
Note: The polling averages used in this post are the averages of the margins of all polls from October 1 to Election Day, unless a trend was observed, in which case only polls from the final 10 days of the election were used.
Update: The title was changed to make it clear that the polls can be wrong either way.