Through the year, Democrats have been winning special elections by an average of 11 points over what the typical partisanship index for that district/state would indicate:
I have a theory on why this is, and it also explains why Biden seems to be underperforming among young people in recent polls:
There’s a segment of very tech savvy, very internet savvy young voters who are overwhelmingly progressive but are skilled enough (and cynical enough) to deliberately avoid polling. Let’s call them Swifties. The Swifties vote at a pretty high rate for young people, and like Taylor Swift, they overwhelmingly vote Democratic. But polling is almost entirely unable to reach them. So polling has to judge young people by the non-Swifties – the young people who answer polling at about the same rate as the rest of the population. These young people are less tech savvy, and less progressive, then the average young person. And so these are swaying the polling by a small but significant enough margin to make polling lean Republican these days, by perhaps as much as 11 point or so.
We’ll see if this is borne out on election day next year, and I don’t have any evidence for it beyond the continual Democratic overperformance in special elections. But I think it fits the facts, at least.