Of likely voters interviewed by SurveyUSA after Trump and Biden debated 09/29/2020 but before Trump had revealed his COVID diagnosis, Biden led Trump by 8 points, 51% to 43%. In the tiny window of time between when Trump acknowledged his illness but before he was hospitalized, Trump closed to within 4 points, Biden 50%, Trump 46. In interviews completed after Trump had been helicoptered to Bethesda, Biden appears to have consolidated support, leading by 16 points among likely voters interviewed most recently, 56% to 40%.
So
Post-Debate, Pre-Covid: Biden +8
Post-Covid, Pre-Hospital, Biden +4
Post-Hospital: Biden +16
Fascinating.
It actually does substantiate the idea that Trump could have gotten a small, if temporary, bump in support from being diagnosed. But then that was more than wiped away once it was apparent he was genuinely ill.