Americans of all stripes have expressed concern over what has appeared to be a lack of mental acuity on the part of President Joe Biden during public events. But it’s what’s been happening behind closed doors at private, invitation-only fundraisers that really has his donors starting to worry. According to Axios, which is not exactly an anti-Biden outlet, donors have been expressing doubts about the president’s ability to mount a full campaign this year, never mind his ability to actually run the country now or for four more years should he win in November. Biden has claimed to feel younger th…