To think, An American pontiff could have a career as Pope , retire and stand for the presidency.
Not sure the RCC would countenance this (I know we are talking theoretically, as I think the RCC do not allow any of their ordained clergy from holding any political office, on the basis that they should only have allegiance to the Holy See.
This is why senior RCC cardinals, archbishops etc have never accepted peerages even once they'd left office as they remain RCC ordained priests.