United States senators serve 6-year terms, although they may seek reelection after each term, and there is no formal or official limit to the number of years or terms a senator may serve. Senators may resign voluntarily or opt to not seek reelection, and they can also be formally expelled by the Senate in the case of wrongdoing; in these cases, two-thirds of the Senate must vote for expulsion, and it is very rare for this process to occur. Each senator serves alongside a fellow senator from the same state, and each state in the union is accorded two Senate seats, as are two unofficial "shadow senators" from the District of Columbia.
Senators are not bound by a term limit, but this does not mean that their terms as senators are necessarily indefinite. Every 6 years, senators must seek reelection against opponents in his or her home state. In some cases, long-term senators who have served for decades are replaced by newly elected representatives, but this is a rare occurrence, and senators typically hold their jobs through successful reelection for a long period of time. Senior senators often run for president, as senators such as John Kerry have done, or they may be appointed to high-ranking federal positions, such as Secretary of State.