Twenty-second Amendment to the United States Constitution
Since coming into force in 1951, the amendment has applied to six presidents who have been elected twice: Dwight D. Eisenhower, Richard Nixon, Ronald Reagan, Bill Clinton, George W. Bush, and Barack Obama. It has also applied to two who entered office intra-term due to their predecessor's death or resignation: Lyndon B. Johnson and Gerald Ford. Johnson became president in November 1963, following the assassination of John F. Kennedy, served out the final of Kennedy's term, and was elected to a full four-year term in 1964. Four years later, he briefly ran for a second full term, but withdrew from the race during the party primaries. Had Johnson served a second full term – through January 20, 1973 – the total length of his presidency would have been ; as it happened, Johnson died two days after this date. Gerald Ford, who became president in August 1974 following the resignation of Richard Nixon, served the final of Nixon's term, and attempted to win a full four-year term in 1976, but was defeated by Jimmy Carter. Johnson was eligible to be elected to two full terms in his own right, as he had served less than two years of Kennedy's unexpired term, whereas Ford was eligible to be elected to only one full term, as he had served more than two years of Nixon's unexpired term.
Dual mandate
Unlike many federations, U.S. states do not generally restrict state or federal officials from seeking office at another level of government without resigning their existing offices first. For example, in the four U.S. presidential elections contested from 1988 to 2000 inclusive, three sitting state governors were nominated for the presidency, these being Michael Dukakis in 1988, Bill Clinton in 1992 and George W. Bush in 2000 (Clinton and Bush were elected president), while in 2016 sitting governor Mike Pence was elected vice president. Elsewhere, serving state officials often seek federal office, one prominent example being Illinois State Senator Barack Obama's election to the United States Senate in 2004 - Obama quickly resigned from the Illinois Senate after being elected to the U.S. Senate despite not being legally required to do so, and served as a U.S. senator until 2008 when he was elected president. Also, it is not uncommon for sitting federal officials to contest election to state offices, although in these cases the office sought is usually one of the state's highest political posts, typically governor - one such recent example being the aforementioned Mike Pence who was a sitting U.S. Representative when he was first elected governor.