The senate model is clearly underestimating Republicans due to reliance on polls, the GOP has outperformed what the polls said in the Senate 4 cycles in a row, 2014, 2016, 2018 and 2020, in all 4 cycles the GOP won more senate seats than most projected, no reason to believe the polls have corrected for the bias.
I know that’s true for 3 of the elections, but are you sure about 2018? I thought the results matched the polls in the Senate then - it was only Democrat hopium that was defeated.
The 2018 senate polls were biased, look at states like Indiana, Missouri, Florida etc, even in many senate races democrats won, they won by smaller margins than the polls said.
https://projects.fivethirtyeight.com/2018-midterm-election-forecast/senate/GOP did better than the median 538 senate forecast.
