At some point I'll write longer on this, but I think part of the reason the Biden-must-go story has been so huge is that a lot of commentators / voters / even members of the press feel angry and betrayed. Many of us largely trusted Biden's team when they insisted he was fine. The debate made very clear that he's not fine. Initially, I think many of us felt some combo of pity for Biden and fear for the future. Now, I think there's rising resentment at being had.
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To be clear, the Biden team could fully Weekend at Bernies him or send three toddlers in a trench coat to the next debate and I would still vote Biden over Donald Trump, and will encourage everyone else to do the same. But like... not ideal.
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And journalists have very much looked into the question of how Biden's age is impacting his abilities, and reported on how voters are worried about it. They've also been accused of making up a non-issue, of being ableist, of facilitating a Trump win, and on and on.
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And in the meantime, Biden's team has made him available for fewer interviews than any president since Reagan. I do think some journalists learned the lessons of the "but her emails" disaster and have tried to moderate on the age question, looking more deeply at the existential threat Trump poses. But now the age issue is fully out in the open and undeniable.