On July 18, 1947, President Harry Truman signed the Presidential Succession Act, which determined the line of succession if the president dies in office or is otherwise unable to perform the duties of president. Almost two decades later, this act became the 25^th Amendment. Since that time, the 25^th Amendment has been invoked six times and is an essential part of the operation of the United States government.  

Prior to the Presidential Succession Act, there were two different plans for succession. Under the Congress of 1792, the President pro tempore of the Senate and then the Speaker of the House would become acting president until a new president was elected. The Presidential Succession Act of 1886 then changed the succession to allow the Secretary of State to become acting president in case the president was removed. In 1947, President Truman needed to appoint a new Secretary of State. Members of Congress became concerned that under the current rules of succession, Truman would be in a position to choose his own successor. They felt that was not true to the democratic process that the framers intended. It was this circumstance that caused Congress to draft a new succession act.  

Under the Presidential Succession Act of 1947, if the President of the United States dies, resigns, or for whatever reason is unable to perform the offices of president, the duties become those of the Vice President, then the Speaker of the House, President Pro Tempore of the Senate, and then Secretary of State. It also indicates that if the vice president leaves office, the president can nominate a new vice president who will need to be confirmed by Congress. The act also allows the president, in cases where he is temporarily unable to perform his duties, to submit written notice to Congress, and the vice president can take their place. This portion of the act has been used in cases of medical procedures during which the president was under anesthesia.  

Following the assassination of President Kennedy, there was confusion about what the proper procedure was to swear in Vice President Johnson as president. This encouraged Congress to ratify the Presidential Succession Act as the 25^th Amendment to the Constitution of the United States.  

Learn more here:

1. https://constitutioncenter.org/the-constitution/amendments/amendment-xxv
2. https://repository.law.umich.edu/cgi/viewcontent.cgi?article=7952&context=mlr
3. https://www.senate.gov/about/officers-staff/president-pro-tempore/presidential-succession-act.htm
4. https://www.usa.gov/presidential-succession
5. https://reagan.blogs.archives.gov/2021/01/08/whos-in-charge-the-25th-amendment-and-president-reagans-assassination-attempt/
6. https://constitutioncenter.org/blog/how-jfks-assassination-led-to-a-constitutional-amendment-2