Twenty-second Amendment to the United States Constitution
The amendment prohibits any individual who has been elected president twice from being elected again. Under the amendment, an individual who fills an unexpired presidential term lasting greater than two years is also prohibited from winning election as president more than once. Scholars debate whether the amendment prohibits affected individuals from succeeding to the presidency under any circumstances or whether it only applies to presidential elections.
I do not choose to run
Some feel that Coolidge was honoring the two-term tradition set by President Washington. While Coolidge held the opinion that a vice president who succeeded to part of another president's term would not violate the rule by then running twice for election in his own right, this tradition was continued through him and in 1952 and 1968, by Presidents Harry S. Truman and Lyndon B. Johnson when the latter chose not to seek another term after disappointing showings in the New Hampshire primary. Both had succeeded to the presidency after the death of their predecessors and had been elected only once. Theodore Roosevelt also did not run in 1908 after his succession to the presidency in 1901 and his election in 1904. However, he ran unsuccessfully in 1912, against his successor, William Howard Taft.