Some of these have been mentioned by others in this thread, but these are the things that I would consider to be red flags:
- Many national polls showed a tightening of the race between mid-October and late October/early November
- Selzer poll
- Relatively weak Democratic performance in Washington's top-two primaries (hat tip to Sean Trende)
- Relatively weak Democratic performance in 2020 special elections (iirc both Trende and 538 had noticed this)
- The Biden and Trump campaigns generally treating the race like it was still competitive (at the time, and arguably even now, it wasn't clear if they really believed it was a closer race than polls showed or if they were simply being cautious/optimistic)
- Biden's fairly weak performance with Hispanic voters in the primaries
One other thing that I think is worth mentioning, and that I still don't completely understand, is the
huge number of people (not all of them conservative Republicans) who thought that a Trump victory was inevitable. Obviously they were wrong, and I think in many cases they were just politically unsophisticated people reading too much into the last election, but I wonder if some of these people were legitimately picking up on something that media/polling wasn't.
