Under Article II, Section 3, of the Constitution, the Senate is made up of two members from each state, each of whom has one vote. Unlike the House of Representatives, in which the entire chamber is up for election every two years, only one-third of the senators are up for reelection every two years.
The Constitution requires that a senator be at least thirty years of age and a U.S. citizen for a minimum of nine years. A senator must make her legal residence in the state that she represents.
The Constitution originally provided for the election of senators by state legislatures. However, the SEVENTEENTH AMENDMENT to the Constitution, adopted in 1913, mandated the election of senators by popular vote. The Senate may punish members for disorderly behavior. With the concurrence of two-thirds of the senators, it can expel a member.
When a vacancy occurs in the representation of any state in the Senate, the governor of that state issues a writ of election to fill the vacancy. The state legislature, however, can empower the governor to make a temporary appointment until the people fill the vacancy through an election.
The vice president of the United States is president of the Senate but has no vote unless the senators are equally divided on a question. His vote breaks the tie.