“Yes, the polls correctly predicted that Joe Biden would win the presidency. But they got all kinds of details, and a number of Senate races, badly wrong. FiveThirtyEight’s polling models projected that Biden would win Wisconsin by 8.3 points; with basically all the votes in, he won by a mere 0.63 percent, a miss of more than 7 points. In the Maine Senate race, FiveThirtyEight estimated that Democrat Sara Gideon would beat Republican incumbent Susan Collins by 2 points; Gideon lost by 9 points, an 11-point miss.”
…
“The theory is that the kind of people who answer polls are systematically different from the kind of people who refuse to answer polls — and that this has recently begun biasing the polls in a systematic way.
This challenges a core premise of polling, which is that you can use the responses of poll takers to infer the views of the population at large — and that if there are differences between poll takers and non-poll takers, they can be statistically “controlled” for by weighting according to race, education, gender, and so forth. (Weighting increases and decreases the importance of responses from particular groups in a poll to better match their share of the actual population.) If these two groups do differ systematically, that means the results are biased.
The assumption that poll respondents and non-respondents are basically similar, once properly weighted, used to be roughly right — and then, starting in 2016, it became very, very wrong. People who don’t answer polls, Shor argues, tend to have low levels of trust in other people more generally. These low-trust folks used to vote similarly to everyone else. But as of 2016, they don’t: they tend to vote for Republicans.”…
“n 2020, Shor argues that the differences between poll respondents and non-respondents have gotten larger still. In part due to Covid-19 stir-craziness, Democrats, and particularly highly civically engaged Democrats who donate to and volunteer for campaigns, have become likelier to answer polls. It’s something to do when we’re all bored, and it feels civically useful. This biased the polls, Shor argues, in deep ways that even the best polls (including his own) struggled to account for.
Liberal Democrats answered more polls, so the polls overrepresented liberal Democrats and their views (even after weighting), and thus the polls gave Biden and Senate Democrats inflated odds of winning.”
https://www.vox.com/policy-and-politics/2020/11/10/21551766/election-polls-results-wrong-david-shor