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.
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.