When President Kennedy was assassinated on November 22, 1963, Vice President Lyndon Johnson took the oath of office aboard Air Force One. But the succession exposed a dangerous gap: with no vice president, the next in line was 71-year-old House Speaker John McCormack, followed by 86-year-old Senate President Pro Tempore Carl Hayden. That crisis led directly to the 25th Amendment, ratified in 1967.
The 25th Amendment settled a question that had been murky since 1841, when John Tyler became the first vice president to assume the presidency after William Henry Harrison's death. Section 1 makes clear that the vice president becomes president -- not just acting president -- if the office is vacated. Section 2 created a process to fill a vice presidential vacancy: the president nominates someone, and both chambers of Congress must confirm. This provision has been used twice -- Gerald Ford replaced Spiro Agnew in 1973, then Nelson Rockefeller replaced Ford in 1974. Beyond the vice president, the Presidential Succession Act of 1947 sets the line: Speaker of the House, Senate President Pro Tempore (currently Senator Chuck Grassley, sworn in January 3, 2025), then Cabinet secretaries starting with the Secretary of State.