Think about it for a second. Is there anything that Robert De Niro can't do? Granted, his recent movie career might now have involved the kind of 'respectable' acting work that he was involved in when he was younger, but that might be a sign that he's overdue to get out of the game. He's a wise and seasoned man now, and he has enough life experience to offer up his services as President of the USA.
We've seen De Niro take on all kinds of roles in his films. The temperament of Travis Bickle from 'Taxi Driver' would be useful in dealing with America's enemies. The shrewdness and cunning from his 'Godfather' role would make him an expert political tactician. Plus, he has A-List star quality. Other world leaders would be awed by his presence the moment he walked into a room.