Trump mentally declined a long time ago, too. One of the differences not really being talked about here between the parties is the loudness of the cult of Trump, the propaganda machine that was at an all-time high across conservative fake news, Russian disinformation, Broligarchy-owned social media, and religious institutions. Remember, even utterly destroying Trump in a one-on-one debate had zero impact on a media apparatus that lives in his attention economy.
A giggling canine once told me...
This isn't the same context. It's not a whataboutism because it is part of the answer. If we're having an honest discussion about the thread topic "Dem post-mortem," then we have to ask the question what was the cause of the Dem loss. To answer the question, we can contrast the Dem performance the last time against the Dem performance in 2020. In doing so, the main difference we can observe is particular groupings of voters who changed votes to Trump. In doing that analysis, it is noted that those groups were looking for economic change and immigration was also an issue, but immigration is also partly economic (i.e. "they took our jobs!!!111"), and so economics is the primary electoral issue that people were paying attention to who flipped. One facet of the flips is perception and opinion and it is necessary to examine where people get their information from. So, to do that one ought to consider information sources typically used by people and of course those include all the things I had listed, but also there was a flip in information sources in that the Broligarchy was no longer neutral but came out swinging for Trump and Trump was very loud, went on Joe Rogan and masculinist podcasts etc. You also had Bezos's Washington Post changing content. I don't expect the latter to have impacted much as the former, but it's an additional example. That attention economy and The Algorithm did not make criticism of Trump (or his mental decline) into memes, even though he has had way more moments of saying ridiculous, unhinged things. Instead, it was all about what he said in a positive light and then moving on to the next, like that he would fix the Price of Eggs on "DAY ONE." It isn't a whataboutism to mention in a post mortem, that the extent of the power of the proaganda machine is so far-reaching, it even takes votes from the traditional base.