Director Otto Preminger offered the role of a Southern senator to Martin Luther King Jr., believing that the casting could have a positive impact (despite the fact that there were no black senators at the time). King declined after serious consideration, as he felt playing the role could cause hostility and hurt the civil rights movement.
The film is based on the Pulitzer Prize-winning novel by Allen Drury, who was a congressional correspondent for The New York Times during the 1950s, while he was writing the book. Nearly every character is based on a real person (Lafe Smith is based on John F. Kennedy; Orrin Knox is based on Robert A. Taft, Fred Van Ackerman is based on Joseph McCarthy and the president is modeled on Franklin D. Roosevelt). Even the blackmailing pf Brig Anderson, and how it's resolved, is based on a real incident. And the Leffingwell nomination is based on the House Un-American Activities Committee investigation of Alger Hiss.
When Allen Drury was writing the novel on which this film is based, John F. Kennedy, upon whom the character Sen. Lafe Smith was based, was a young senator with ambitions to be President. When the movie came out Kennedy was President, and Lafe Smith was played by Peter Lawford who was, at that time, married to Kennedy's sister Patricia Kennedy.
Burgess Meredith, as Herbert Gelman, testifies against Leffingwell at the latter's confirmation hearing, claiming that the two of them were members of a Communist cell. In real life, Meredith was himself named an "unfriendly witness" by the House Un-American Activities Committee, which nearly ruined his career. Will Geer, who plays the Senate minority leader, was also blacklisted for refusing to name names before the same Committee.
The man who is seen turning down a drink from a passing waiter is then-U.S. Senator and future Democratic presidential candidate Henry Jackson (aka "Scoop" Jackson), who appears uncredited. At a special private preview of the film for members of Congress, the sight of Jackson refusing a drink drew gales of laughter from his colleagues.
There is a scene showing Sen. Seabright 'Seab' Cooley and Senate Majority Leader Bob Munson driving up to and talking inside a residential apartment building in which both of them live (in separate apartments). The "apartment building" is actually the original section of The Marriott Wardman Park Hotel, now called The Wardman Tower. The hotel and tower still exist, on the corner of Connecticut Avenue and Woodley Road NW, and is the largest, and one of the most historic, hotels within the city limits of Washington, DC.
Peter Lawford was cast because Otto Preminger valued his access to the Kennedy family. Lawford was John F. Kennedy's brother in law, and JFK was also a massive movie buff who loved having a film crew around the White House.
The trivia items below may give away important plot points.
The blackmail attempt is based on the case of Wyoming Sen. Lester C. Hunt, who was blackmailed by members of the Republican Party. Hunt was told by Sen. Styles Bridges that if he ran for re-election that November, the details of his son's arrest (for soliciting prostitution from a male undercover officer) would end up "in every mailbox in Wyoming". Hunt eventually agreed to step down, but 11 days later committed suicide in the Capitol.
The film plot centers on a terminally ill President trying to preserve his foreign policy agenda by appointing a Secretary of State he has confidence in. Technically, the new President can fire the Secretary of State and appoint whoever he likes. However, the Secretary of State is the senior member of the cabinet. The death of a President often requires the new President to provide a sense of continuity to alleviate anxiety in the public. That is why of the 8 Vice Presidents who have succeeded to the Presidency (7 by death, 1 by resignation), 6 of them have kept the sitting Secretary of State for some or all of their terms.
In the novel, the president is behind the attempt to blackmail Senator Annderson and Senator Van Ackerman is his cat's paw. In the film, Senators Munson and Anderson suspect the president but accept his denial.