The discourse is suddenly alive with revived talk of how the public turned against Joe Biden because of his mental incapacity. In particular, they hated being gaslit: told he was fine when the evidence of our eyes told us otherwise. However, there are some problems with this theory. Here's the first one:
Biden's approval rating dropped to under 40% by the end of 2021 and never budged by more than a couple of points after that. But that was long before serious concerns about his fitness began to stir. The evidence suggests the public turned against him because of inflation and Afghanistan and not much else.
Other polls had long showed slight unease with Biden's age, but again, that barely changed during his entire presidency. It was there from the start and never got worse until the very end.
Second, all the inside dope we're aware of suggests Biden was mentally fine all the way through the start of 2024. Only at that point did insiders start to notice deterioration, and Biden put their doubts temporarily to rest with his strong State of the Union address in February.
Only the next few months were dicey, finally ending with a decisive vote of no-confidence only after Biden's disastrous debate performance in late June. Biden's approval did drop a bit during this period, but then rebounded when he pulled out of the race. This is a little hard to parse. Did voters continue to harbor a small amount of resentment over the "Biden charade" or was everything forgiven and forgotten after Kamala Harris took over the reins?
It's hard to say with any precision, but the evidence suggests that Biden's fitness had, at most, only a tiny impact on the race.