So
538 has their polling and forecast post-mortem up and while I agree with the bottom line that the polls didn't have much of a bias overall, I think the one thing they keep missing is that there
are consistent biases in specific
states. Upon examining 2022 results, I found that these patterns haven't reversed completely. The big peculiar thing was that
swing states had a significant Republican bias this year (even swing states that previously had a huge Democratic bias, like Wisconsin). But aside from that anomaly, which might be explained by the flood of junk Republican polls, the other patterns are similar to previous cycles: notably, the fact that Republicans massively overperform their polls in solidly Republican states (Democrats tend to do the same in a few very Democratic states, but not as consistently).
Overall, I've decided to calculate the average polling miss by state over the past 4 cycles. That gives us 2 presidential, 4 senatorial and 2 gubernatorial rounds of elections. Since the Senatorial and Gubernatorial series don't cover every states, I've imputed missing values using a regression model with the presidential data. This is what the patterns look like:
States where the average polling error is less than 1 point are in grey. Then states where Republicans overperform (ie, there is a Democratic polling bias) by more than 1 are in blue, and those where Dems overperform (GOP polling bias) are in red. As you can see, there is a huge swathe of the country where Republicans beat their polls, sometimes by downright comical margins: over 10 points in WV, Idaho and the Dakotas. Mainly, those are states that rarely if ever have competitive races, so it makes little practical difference. However, they also critically include a lot of Midwestern swing states. This is where Democrats got burned in 2016 and to some extent 2018 and 2020. This is also, crucially, where the polling bias reversed itself in 2022 (Democrats overperformed by 3 points in WI, 4 points in PA and 5 points in MI), but overall the average still tends to favor Republicans there. Time will tell if this is a long-term reversal or a fluke. Conversely, California saw very little polling bias in 2020 and 2022, so its history of Democratic overperformance (which was huge in 2016 and 2018) might be over. Of course, a good chunk of the country, including many newer swing states like GA, AZ and NV, see very little if any polling bias.
Going forward, I will adjust my state-level predictions using these polling bias figures, and we'll see if they help me beat the 538 forecast or not. This year it didn't help much, given the aforementioned overperformance in swing states, but if there's anything 538 taught me is to trust averages over one-time blips.