The Senate is one of the two chambers of the U.S. Congress that possess the legislative power of the federal government. The Senate, which now has one hundred members, has two senators from each state. Until 1913, senators were elected by their state legislatures. But with the adoption of the 17th Amendment, senators have been elected directly by the people of each state.
There are several exclusive requirements to be a senator: he or she must be over thirty years of age, must have been an American citizen for at least nine years, and must live in the state he or she represents. Senators can serve for an unlimited number of six-year terms. Senatorial elections are held on a staggered basis so that one-third of the Senate is elected every two years.
If a senator leaves office before the end of his or her term, the 17th Amendment now provides that the governor of his or her state sets the time for a new election. The state legislature may authorize the governor to temporarily fill the vacant seat. – The United States Constitution, What It Says, What It Means, A Hip Pocket Guide