A majority of respondents were telling pollsters they were better off than they were four years ago and that they approved of the president's handling of the economy. The environment for a D landslide is when an incumbent D has those numbers, or an R has much worse numbers. https://twitter.com/AlxThomp/status/1324395162353704962

Biden has massively outperformed the fundamentals. Of course, there are a lot of reasons for that. But still, sometimes Democrats will go into elections with better fundamentals than this.

Democrats believed their own talking points about the economy not being so good in 2018 or 2019 -- Dem debate answers about this were super lame -- and in 2020, voters continued to give Trump credit, seeing the economic crisis as exogenous. They did not have an economy advantage.