Legend suggests that the future Republican American President Ronald Reagan, late in his previous career as a Hollywood actor, was rejected for a role in this film because a studio executive at United Artists didn't think he had "that presidential look". However, Gore Vidal, in one of his several essays attacking Reagan's presidency, says that Reagan was actually considered for the role of Joe Cantwell during preparations for the first Broadway production of his original play in 1960. (Frank Lovejoy eventually played Cantwell on stage). 1964, the year the film version appeared, was the year Reagan decisively left acting for politics, so Vidal's version of the story is the more likely.
Screenwriter and the film's source playwright Gore Vidal cheerfully admitted that he meant the character of William Russell (Henry Fonda) to remind people of Adlai Stevenson and that Joe Cantwell (Cliff Robertson) was based on Richard Nixon. Stevenson and Nixon were of different American political parties, Democrat and Republican respectively. Similarly, the character of former President Art Hockstader played by Lee Tracy, bore resemblances to both former Republican U.S. President Dwight D. Eisenhower and former Democrat U.S. President Harry S. Truman.
Actor Lee Tracy reprises the role of former President Art Hockstader which he had created on Broadway and for which he was nominated for the 1960 Tony Award for Best Actor, losing to co-star Melvyn Douglas, who played the William Russell character on stage, the latter of whom is portrayed by Henry Fonda in this movie. Tracy in turn was also nominated for an Academy Award for the Art Hockstader role.
One of the filming locations was the Ambassador Hotel in Los Angeles, California. Shortly after midnight on June 5, 1968, Robert F. Kennedy was assassinated there after addressing supporters on his victory in California's Democratic presidential primary, which had put him in a solid position to win the party's nomination at the national convention in Chicago that summer. The building is now demolished.
Gore Vidal was once close to the Kennedy family, and an actual genetic cousin of Jacqueline Kennedy Onassis. However, it is alleged that the release of this movie caused his friendship with the Kennedy family to cool.
Gore Vidal: Uncredited, the film's source playwright and screenwriter as a delegate at the party convention.