(5) Senator Joseph Clark, Jr. of Pennsylvanis https://en.wikipedia.org/wiki/Joseph_S._Clark_Jr. becomes POTUS--maybe he is elected VP as Humphrey's running mate in 1968, Humphrey wins, Humphrey dies in office, and Clark is elected POTUS in his own right in 1976. He is still POTUS when Joe Clark become Prime Minister of Canada in 1979... https://en.wikipedia.org/wiki/Joe_Clark