So the basic story is that, particularly after Covid-19, Democrats got extremely excited, and had very high rates of engagement. They were donating at higher rates, etc., and this translated to them also taking surveys, because they were locked at home and didn’t have anything else to do. There’s some pretty clear evidence that that’s nearly all of it: it was partisan non-response. Democrats just started taking a bunch of surveys [when they were called by pollsters, while Republicans did not].

Just to put some numbers on that, if you look at the early vote results, and compare it with the cross tabs of what public polls said early voters were going to be, it’s pretty clear that early voters were considerably less Democratic than people thought. Campaign pollsters can actually join survey takers to voter files, and starting in March, the percentage of our survey takers who were, say, ActBlue donors, skyrocketed. The average social trust of respondents went up, core attitudes changed — basically, liberals just started taking surveys at really high rates. That’s what happened…

If you match to vote history, literally 95 percent of people who answer phone surveys vote. That’s the problem with “likely voter screens” [which try to improve polls by limiting them to the likeliest respondents to vote]. If you restrict to people who have never voted in an election before, 70 percent of phone survey takers vote. If you restrict to people who say they will definitely not vote, 76 percent of those people vote.

Normally that doesn’t matter, because political engagement is actually not super correlated with partisanship. That is normally true, and if it wasn’t polling would totally break. In 2020, they broke. There were very, very high levels of political engagement by liberals during Covid. You can see in the data it really happened around March. Democrats’ public Senate polling started surging in March. Liberals were cooped up, because of Covid, and so they started answering surveys more and being more engaged.