Were there any red flags that indicated the race would be closer than polling suggested?

[Pericles]

One important thing was that the special elections didn't show great swings to the Democrats like they did before the 2018 midterms, but did actually indicate a more neutral environment. Mike Garcia's 10 point win was particularly alarming. Still, it was not too hard to just handwave it by saying that even if that was meaningful, public opinion by election time had swung against Trump. Unfortunately, it just wasn't true.

With the House, I did always have a niggling doubt that people were going a bit wild with their predictions. If the Democrats were supposed to win the House popular vote by about the same or less than in 2018, how were they supposed to improve in pretty much every swing seat and make like 20 net gains? Democrats did distribute their vote better in the House than in 2018. However, on a uniform swing, Democrats would have only gotten a few more seats than in 2018, not even 240, if the generic ballot was exactly right. And even if they had gotten a D+8.6% margin, they would have only gotten 241 seats (disturbingly, Republicans got that exact seat total in 2016 with a 1.1% popular vote win).