Why is it mostly moderates calling for Biden to step aside, whereas the left wing mostly supports him continuing the race?

I'm not sure that your political alignment has anything to do with it. I'm British, so it makes no difference to me who the President is.

The question I have is:- At which point do you consider a President to be mentally incompetent??? I look at both Trump and Biden - the two candidates you have to choose between and I wonder which of the two has the least cognitive decline???

Neither of them seem to even know what day of the week it is, never mind what the national defense policy is. Trump rewrites the geography of America to suit himself... Biden has no clue who he's talking to... when do you say "We want a President who is capable of understanding what they're agreeing to???

Am I the only one who can see just how dangerous it could be to have someone who lacks the mental capacity in possession of nuclear weaponry that has the potential to make Chernobyl look like a fire pit in your back garden???