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Existing comment: Vice President Moves Up
Who's next in line when the President and Vice President die or resign? For a total of 38 years, the Vice Presidency has been vacant when the VP died or became President. When Lyndon Johnson became President after John Kennedy's assassination in 1963, the tragedy galvanized support for the 25th Amendment, which provided for the appointment of a Vice President.

Lyndon Baines Johnson taking the Presidential Oath of Office Aboard Air Force One, November 22, 1963, John F. Kennedy Presidential Library
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