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 crosstabs 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.