If Biden is so fragile, who would want him to be POTUS? If he can't think straight, how can he lead? Putin, or anyone with a full deck, would destroy him...
They think a strong anti-trump voter bloc will vote for a senile old fool rather than vote for Trump. Plus Joe is a vehicle for whoever the Dems really want to run the country. He wont make it six weeks into office if somehow he beats trump, the 25th amendment will be used