I remember when Biden was campaigning at the Iowa state fair back in 2020. It was one of the first times that he had been speaking at a public event since the end of the Obama administration. He was a ghost of his former self. It was really shocking. Biden has always been a gaff machine, but he always had a very strong presence when speaking publicly. Biden looked like he was recovering from being lost at sea for six months. Bernie is almost a year older and looked like he was 40 years younger compared to Biden.
I am not sure Biden had dementia or was just not aging well. But he clearly was losing the energy and cognitive ability needed to be president. And that decline just continued until it was revealed at the debate with Trump.
The DNC corporate Dems and Biden's long time advisors were just convinced from polling that only an older white Dem could beat Trump and any combination of youth, POC, female or progressive would be doomed against Trump. But they underestimated Biden's decline. Reagan did show signs of early onset of Alzheimer's during his second term. But it was hard to tell whether he was in cognitive decline or was just the same Reagan who often struggled with the press when he was off script. But Reagan was still pretty good in public appearances and was just as crisp when he was reading his script. Biden was so bad at any sort of public appearance that he had to basically be kept hidden for the majority of his term. That meant that the Dems lost the narrative going in to 2024 because they basically had no president out there arguing for why his administration was a success. Trump then captured the narrative and won.