The U.S. Senate, along with the U.S. House of Representatives, are the two chambers of the U.S. Congress who together make up the legislative branch of the government. Th are 100 senators, two from each U.S. state. In November of every even-numbered year, there is an election to fill as many as one-third of the seats in the Senate. Once elected, each senator serves a six-year term, but elections are staggered so all terms don't begin and end at the same time.

Balance of Power

The authors of the U.S. Constitution designed the Senate to protect the rights of states by giving each state the same voice regardless of its size. In the House of Representatives, seats are allotted to states according to population. Since larger states have more representatives than smaller ones, the interests of states with dense urban populations tend to dominate. In the Senate, however, each state has two senators and each senator has one vote. When the Constitution was originally ratified, senators were elected by state legislatures. The purpose was, in part, to make state lawmakers feel connected to the federal government -- but also to ease the minds of state political leaders who feared a strong central government would trample state rights.

Continuity by Thirds

The stability of the Senate would be hampered if it were possible for the entire membership to change all at once. To provide for both continuity and rotation, the authors of the Constitution decided that one-third of the senators would be elected every two years, leaving the other two-thirds in office.

The day before the first senate session, a special committee had divided the senators, ensuring each group included members from all regions of the country and only one from each state. On May 15, 1789, one senator from each group pulled a slip of folded paper from a box that had a number written on it. Senators in class number one made the sacrifice of serving initial terms only two years long, while senators in class number two served four-year terms. This allowed terms to be staggered. As new states were added, the same basic process was used.

Six-Year Terms

Senators were given six-year terms of office. These lengthy terms -- three times as long as those of House members -- were intended to elevate senators above short-term politics and give the legislative body more stability.

By the People

After the Civil War, many state legislatures were beset by cronyism, bribery and corruption. The push began for direct popular election of senators, whom heretofore had been nominated and selected by the Senate. By 1912, 29 states were selecting senators using public state referenda either in primary or general elections. In 1913, the 17th Amendment to the Constitution was ratified, allowing for the direct popular election of senators.