After Joe Biden's disastrous CNN debate, he lost a grand total of two points of support in the You Gov weekly tracking poll. Trump gained nothing.

Among independents Biden lost four points and Trump, remarkably, lost one point. Their support mostly went to RFK Jr. and Jill Stein. This suggests that Trump really does have a ceiling on his support.

On average, other polls also show Biden losing a net of 2-3% after the debate. This is remarkably little, probably due to a combination of low viewership and high partisanship.

https://jabberwocking.com/joe-biden-lost-about-two-points-of-support-after-the-cnn-debate/