2004 United States Senate elections

The 2004 United States Senate elections were held on November 2, 2004, with all Class 3 Senate seats being contested. They coincided with the re-election of George W. Bush as president and the United States House elections, as well as many state and local elections. Senators who were elected in 1998, known as Senate Class 3, were seeking re-election or retiring in 2004.

Republicans won six seats but lost two themselves, giving them a net gain of four seats. Five of the six gains came from southern states. Democrats Fritz Hollings of South Carolina, John Breaux of Louisiana, Bob Graham of Florida, and Zell Miller of Georgia chose to retire, while John Edwards of North Carolina chose to run for the Democratic nomination for president, and was subsequently selected as the Democratic nominee for vice president. They were succeeded by Republicans Jim DeMint, David Vitter, Mel Martinez, Johnny Isakson, and Richard Burr, respectively.

In South Dakota, Republican John Thune defeated the incumbent Senate minority leader Tom Daschle, marking the first and only time since 1952 that a sitting party leader lost re-election, as well as the only time that person was the minority leader. Republican senator Peter Fitzgerald of Illinois chose not to run for re-election, and Democrat (and future president) Barack Obama won in a landslide, becoming the Senate's only Black member and only the third popularly elected in American history and only the third elected since Reconstruction. Also, Republican senator Ben Nighthorse Campbell of Colorado chose not to run for re-election and Democrat Ken Salazar won the open seat.

This was the third consecutive election cycle for Senate Class 3 where the Democrats either broke even or lost seats. This also marked the first time since 1980 in which a presidential candidate from either party won with coattails in the Senate. Additionally, these election results are tied with those of 1996 and 1998 as the highest number of Senate seats that the Republicans have held since 1920. Republicans would not make a net gain of seats during a presidential election year again until 2024. This is the last time both major parties were on the general election ballot in every race.