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).
List of Presidents of the United States by time in office
William Henry Harrison spent the shortest time in office, and Franklin D. Roosevelt spent the longest. Roosevelt is the only president to have served more than two terms. Following ratification of the Twenty-second Amendment in 1951, presidents—beginning with Dwight D. Eisenhower—have been ineligible for election to a third term or for election to a second full term after serving more than two years of a term to which some other person was elected president. The amendment contained a grandfather clause that explicitly exempted the incumbent president, then Harry S. Truman, from the new term limitations, which first applied to Dwight D. Eisenhower.