Pollster David Shor explains to Vox why the public polling was off this year:
“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.”

