OK, here's an idea --
After finishing the film version of Mister Roberts he decides to take a furlough from movies and theater, maybe only intending it to be a year.
He gets involved in a high profile way in the 1956 election
1958 he runs for Governor of California -- is elected, and re-elected 1962
After two failed bids for the Democratic nomination for President in 1960 and 1968, Fonda is elected President in 1972
How's that?