‘If he is found to be ineligible after Jan 20th, who will impeach him? There is no way for the SCOTUS to declare him not President (I think?), so what happens then?’
After Jan 20 he can be ruled ineligible and impeached by Congress. Before Jan 20, if ruled ineligible the 20th Amendment would kick in and Biden would be the acting President-Elect
“After Jan 20 he can be ruled ineligible and impeached by Congress.”
Do you think the Democrats would impeach him?