The U.S. Constitution gives each house of Congress the power to be the judge of the "elections, returns, and qualifications of its members." Since 1789, the Senate has closely guarded this prerogative, and has developed its own procedures for judging the qualifications of its members in contested elections.

Originally, senators were elected by the state legislatures. The Seventeenth Amendment to the Constitution, ratified in 1913, established direct popular election of senators. Under both the new and the old system, the Constitution permits state governors to make temporary appointments to fill Senate vacancies.

Election disputes reviewed by the Senate have fallen into several general categories. During the sectional conflicts prior to the Civil War, and in the war's aftermath as former Confederate states rejoined the Union, questions arose regarding the state legislatures' legal authority to elect. Later in the 19th century and into the early 20th century, corrupt use of money in elections became an issue. Since 1913, campaign expenditures, demands for recounts in close elections, and complaints of election irregularities or fraud have constituted the dominant issues.

Election Laws 
Senate Procedure in Contested Elections
Types of Election Cases
Vacancies

1. Seat was declared vacant, but Harlan was immediately reelected and seated.
2. Resigned before Senate could vote to void election.
3. Resigned; reelected the next year and seated.
4. Brookhart was initially seated but was later unseated by the Senate and Steck seated in his place. This is the only occasion to date in which the Senate has actually reversed the results of an election, unseated a senator, and seated the challenger.
5. Cutting died May 6, 1935; on June 4, 1935, the Senate agreed with a committee recommendation to dismiss the contest. Chavez was appointed to fill the vacancy.
6. Bilbo died on August 21, 1947, before the Senate reached a decision.
7. This contest ended in deadlock; a new election was held.
6. Bilbo died on August 21, 1947, before the Senate reached a decision.
8. Following a long contest over the election, including a lengthy recount of ballots, the Minnesota state supreme court decided the election in Franken’s favor. Franken took the oath of office on July 7, 2009.

Source: U.S., Congress, Senate, Senate Election, Expulsion, and Censure Cases from 1793 to 1972, S. Doc. 92-7, 92d Cong., 1st sess., 1972.