Aside from the fact that Biden’s cognitive decline is visible to the naked eye and it is incredibly reckless and repressive to demand that it be supressed, these concerns were first raised not by Trump operatives nor by Sanders supporters, nor were they first raised within the last several weeks. Quite the opposite is true: they were raised repeatedly over the last year principally by Democratic Party officials and their most loyal allies in the media.
One of the earliest mainstream figures to raise serious questions about Biden’s cognitive decline was MSNBC’s Andrea Mitchell immediately before the June, 2019 presidential debate. The long-time reporter, not exactly known as a Trump sympathizer to put that mildly, went on air amidst a series of rambling, incoherent Biden appearances and said: “the question is, does he still have his stuff? How sharp is he?,” prompting Chris Matthews to immediately understand her point and reply: “You know you’re answering the question with the question.”
One of the most explicit and direct attacks on Biden’s cognitive fitness to run for President and to serve came from Democratic Senator Cory Booker (now a Biden endorser). After a CNN presidential debate in September — in which Biden had bizarrely rambled about how the key to solving the problems in African-American communities was having parents play “the record player” for their children, and during which Castro had pointedly accused Biden of having forgotten what he had said just moments earlier — Booker unflinchingly suggested Biden lacked the ability to endure the rigours of the campaign (the full Booker interview is here):