Twenty-second Amendment to the United States Constitution
Some argue that the 22nd Amendment and 12th Amendment bar any two-term president from later serving as vice president as well as from succeeding to the presidency from any point in the presidential line of succession. Others contend that the original intent of the 12th Amendment concerns qualification for "service" (age, residence, and citizenship), while the 22nd Amendment, concerns qualifications for "election", and thus (strictly applying the text) a former two-term president is still eligible to "serve" as vice president (neither amendment restricts the number of times an individual can be elected to the vice presidency), and then succeed to the presidency to serve out the balance of the term (though prohibited from running for election to an additional term).
Acting President of the United States
Presidential succession is referred to multiple times in the U.S. Constitution, as well as the Twentieth Amendment and Twenty-fifth Amendment. The Vice President is the only officeholder named in the Constitution as a presidential successor. The Article II succession clause authorizes Congress to designate which federal officeholders would accede to the presidency in the event that the vice president were unable to do so, a situation which has occurred on three separate occasions. The current Presidential Succession Act was adopted in 1947 and last revised in 2006. The order of succession is as follows: the Vice President, the Speaker of the House of Representatives, the President pro tempore of the Senate, and then the eligible heads of the federal executive departments who form the president's Cabinet, beginning with the Secretary of State.