• Ideological non-response bias. Essentially, white conservatives disproportionately opted-out of their polls, while liberal voters disproportionately opted-in;
• Geographic non-response bias. Respondents living in strongly pro-Trump zip codes were less likely to respond to their polls—even when controlling for partisanship and demography;
Basically, these findings dove-tail with our own view that a widespread lack of trust (in polling, the media and other institutions) resulted in the underrepresentation of key voting groups in most 2020 presidential polling. The fact that most of this was found among white conservative voters incorrectly led to a "consensus" that Biden was ahead by 9-10 points, when in fact it ended up closer to 5-6 points.
We have argued for assessing this issue through a battery of questions that can be expressed as a "Trust Index". If trust metrics are disproportionately askew among certain partisan or ideological groups, adjustments can be made in vote projections. Organizations like Data For Progress (and others) should be praised for examining this issue. It's an on-going exercise that the entire polling industry needs to be committed to.