2018 United States Senate elections

The 2018 United States Senate elections were held on November 6, 2018. Among the 100 seats, the 33 of Class 1 were contested in regular elections while 2 others were contested in special elections due to Senate vacancies in Minnesota and Mississippi. The regular election winners were elected to 6-year terms running from January 3, 2019, to January 3, 2025. Senate Democrats had 26 seats up for election (including the seats of 2 independents who caucus with them), while Senate Republicans had 9 seats up for election.

To maintain their working majority of 50 senators and their party's vice president's tie-breaking vote, Republicans could only afford a net loss of 1 seat in these elections. The Republicans had a 52–48 majority after the 2016 elections, but they lost a seat in Alabama in December 2017 after Jeff Sessions resigned to become U.S. attorney general and Doug Jones, a Democrat, won in the subsequent special election. Three Republican-held seats were open as a result of retirements in Tennessee, Utah and Arizona. Although every Democratic incumbent ran for re-election, Democrats faced an extremely unfavorable map, defending 26 seats, of which 10 were in states won by Donald Trump in the 2016 presidential election, and 5 of those where Trump had won by more than 10%. Republicans, however, only had to defend 9 seats, of which only 1 was in a state won by Hillary Clinton in 2016.

The Republicans increased their majority by defeating Democratic incumbents in Florida, Indiana, Missouri, and North Dakota; and holding the open seats in Tennessee and Utah. In contrast, Democrats won 2 Republican-held seats, defeating an incumbent in Nevada and winning the open seat in Arizona.

The results for this election cycle were the only significant gains made by the Republicans in what was otherwise characterized as a "blue wave" election. The Republican gains in the Senate and the Democratic gains in the House marked the first mid-term election cycle since 1970 in which the president's party made net gains in one chamber of Congress while suffering net losses in the other, which also occurred in 1914, 1962, and 2022. This was the first mid-term election cycle since 2002 in which any incumbents of the non-presidential party lost re-election. The number of defeated non-presidential party incumbents (4) was the most since the 1934 mid-terms. As of February 2024, this is the last time Republicans either won control of the Senate and/or made net gains in the chamber.