The congressional generic ballot is one of the few indicators with a long history of non-presidential vote outcomes we have. I’ll look at how good a predictor it is at another time. Here is simply a review of the trends over each election cycle. The chart above shows each cycle since 1999-2000 until the current cycle, as of Oct. 5, 2021.

And for the obsessive, here is a chart of each cycle since 1945-46. It Is unreadable without zooming in, so download it and zoom as you wish.

The .pdf version is best for zooming in. You can download it here.

Updated 2021-10-06 to correct errors in 1981-82 chart.Thanks to @SteveEvets8 for spotting the errors.