The 1998 United States Senate elections were held on November 3, with the 34 seats of Class 3 contested in regular elections. This was seen as an even contest between the Republican Party and Democratic Party. While the Democrats had to defend more seats up for election, Republican attacks on the morality of President Bill Clinton failed to connect with voters and anticipated Republican gains did not materialize. The Republicans picked up open seats in Ohio and Kentucky and narrowly defeated Democratic incumbent Carol Moseley Braun, but these were cancelled out by the Democrats' gain of an open seat in Indiana and defeats of Republican Senators Al D'Amato and Lauch Faircloth. The balance of the Senate remained unchanged at 55–45 in favor of the Republicans.
The 1990 United States Senate elections were held on Tuesday, November 6, 1990, with the 33 seats of Class 2 contested in regular elections. Special elections were also held to fill vacancies. The Democratic Party increased its majority with a net gain of one seat from the Republican Party. The election cycle took place in the middle of President George H. W. Bush's term, and, as with most other midterm elections, the party not holding the presidency gained seats in Congress. This was the last election cycle until 2022 where only one U.S. Senate seat flipped parties.
The 1988 United States Senate elections were elections for the United States Senate. Held on November 8, the 33 seats of Class 1 were contested in regular elections. In spite of the Republican victory by George H. W. Bush in the presidential election, the Democrats gained a net of one seat in the Senate. Seven seats changed parties, with four incumbents being defeated. The Democratic majority in the Senate increased by one to 55–to–45.
The 1986 United States Senate elections were elections for the United States Senate. Held on November 4, in the middle of Ronald Reagan's second presidential term, the 34 seats of Class 3 were contested in regular elections. The Republicans had to defend an unusually large number of freshman Senate incumbents who had been elected on President Ronald Reagan's coattails in 1980. Democrats won a net of eight seats, defeating seven freshman incumbents, picking up two Republican-held open seats, and regaining control of the Senate for the first time since January 1981. This remains the most recent midterm election cycle in which the sitting president's party suffered net losses while still flipping a Senate seat.
The 1982 United States Senate elections were held on November 2, 1982. They were elections for the United States Senate following Republican gains in 1980. The 33 Senate seats of Class 1 were up for election in 1982. A total of four seats changed hands between parties, with Democrats winning seats in New Jersey and New Mexico, and Republicans taking seats in Nevada and the seat of the lone independent, Senator Harry Byrd Jr., in Virginia. Democrats made a net gain of one seat bringing them to 46 seats, while Republicans stayed at 54 seats for a majority. However, the Democratic gain in New Jersey replaced a Republican that had been appointed earlier in the year.
The 1980 United States Senate elections were held on November 4, coinciding with Ronald Reagan's victory in the presidential election. The 34 Senate seats of Class 3 were contested in regular elections. Reagan's large margin of victory over incumbent Jimmy Carter gave a huge boost to Republican Senate candidates, allowing them to flip 12 Democratic seats and win control of the chamber for the first time since the end of the 83rd Congress in January 1955.
The 1978 United States Senate elections were held on November 7, in the middle of Democratic President Jimmy Carter's term. The 33 seats of Class 2 were contested in regular elections. Special elections were also held to fill vacancies.
The 1976 United States Senate elections was an election for the United States Senate. Held on November 2, the 33 seats of Class 1 were contested in regular elections. They coincided with Democrat Jimmy Carter's presidential election and the United States Bicentennial celebration. Although almost half of the seats decided in this election changed parties, Carter's narrow victory did not provide coattails for the Democratic Party. Each party flipped seven Senate seats, although, one of the seats flipped by Democrats was previously held by a Conservative.
The 1974 United States Senate elections were held on November 5, with the 34 seats of Class 3 contested in regular elections. They occurred in the wake of the Watergate scandal, Richard M. Nixon's resignation from the presidency, and Gerald Ford's subsequent pardon of Nixon. Economic issues, specifically inflation and stagnation, were also a factor that contributed to Republican losses. As an immediate result of the November 1974 elections, Democrats made a net gain of three seats from the Republicans, as they defeated Republican incumbents in Colorado and Kentucky and picked up open seats in Florida and Vermont, while Republicans won the open seat in Nevada. Following the elections, at the beginning of the 94th U.S. Congress, the Democratic caucus controlled 60 seats, and the Republican caucus controlled 38 seats.
The 1972 United States Senate elections were held on November 7, with the 33 seats of Class 2 contested in regular elections. They coincided with the landslide re-election of Republican President Richard Nixon. Despite Nixon's landslide victory, Democrats increased their majority by two seats. The Democrats picked up open seats in Kentucky and South Dakota, and defeated four incumbent senators: Gordon Allott of Colorado, J. Caleb Boggs of Delaware, Jack Miller of Iowa, and Margaret Chase Smith of Maine. The Republicans picked up open seats in New Mexico, North Carolina, and Oklahoma, and defeated one incumbent, William B. Spong Jr. of Virginia.
The 1970 United States Senate elections was an election for the United States Senate. It took place on November 3, with the 33 seats of Class 1 contested in regular elections. Special elections were also held to fill vacancies. These races occurred in the middle of Richard Nixon's first term as president. The Democrats lost a net of three seats, while the Republicans and the Conservative Party of New York picked up one net seat each, and former Democrat Harry F. Byrd Jr. was re-elected as an independent.
The 1968 United States Senate elections were elections for the United States Senate. Held on November 5, the 34 seats of Class 3 were contested in regular elections. They coincided with the presidential election of the same year. The Republicans picked up five net seats in the Senate. This saw Republicans win a Senate seat in Florida for the first time since Reconstruction.
The 1964 United States Senate elections were held on November 3. The 33 seats of Class 1 were contested in regular elections. Special elections were also held to fill vacancies. They coincided with the election of President Lyndon B. Johnson by an overwhelming majority, to a full term. His Democratic Party picked up a net two seats from the Republicans. As of 2023, this was the last time either party has had a two-thirds majority in the Senate, which allowed the Senate Democrats to override a veto, propose constitutional amendments, or convict and expel certain officials without any votes from Senate Republicans. However, internal divisions would have prevented the Democrats from having done so. The Senate election cycle coincided with Democratic gains in the House in the same year.
The 1958 United States Senate elections were elections for the United States Senate which occurred in the middle of President Dwight D. Eisenhower's second term. Thirty-two seats of Class 1 were contested in regular elections, the new state of Alaska held its first Senate elections for its Class 2 and 3 seats, and two special elections were held to fill vacancies.
The 1954 United States Senate elections was a midterm election in the first term of Dwight D. Eisenhower's presidency. The 32 Senate seats of Class 2 were contested in regular elections, and six special elections were held to fill vacancies. Eisenhower's Republican party lost a net of two seats to the Democratic opposition. This small change was just enough to give Democrats control of the chamber with the support of an Independent who agreed to caucus with them, he later officially joined the party in April 1955.
The 1936 United States Senate elections coincided with the reelection of President Franklin D. Roosevelt. The 32 seats of Class 2 were contested in regular elections, and special elections were held to fill vacancies. The Great Depression continued and voters backed progressive candidates favoring Roosevelt's New Deal in races across the country. The Democrats gained 5 net seats during the election, and in combination with Democratic and Farmer–Labor interim appointments and the defection of George W. Norris from the Republican Party to become independent, the Republicans were reduced to 16 seats. Democrats gained a further two seats due to mid-term vacancies. The Democrats' 77 seats and their 62-seat majority remain their largest in history.
The 1932 United States Senate elections coincided with Democrat Franklin D. Roosevelt's landslide victory over incumbent Herbert Hoover in the presidential election. The 32 seats of Class 3 were contested in regular elections, and special elections were held to fill vacancies.
The 1916 United States Senate elections were elections that coincided with the re-election of President Woodrow Wilson. This was the first election since the enactment of the Seventeenth Amendment that all 32 Class 1 Senators were selected by direct or popular elections instead of state legislatures. Republicans gained a net of two seats from the Democrats, and then a furthered seat through mid-term vacancies thereby reducing Democrats to a 53-43 majority.
The 1910–11 United States Senate election were held on various dates in various states. As these U.S. Senate elections were prior to the ratification of the Seventeenth Amendment in 1913, senators were primarily chosen by state legislatures. Senators were elected over a wide range of time throughout 1910 and 1911, and a seat may have been filled months late or remained vacant due to legislative deadlock. However, some states had already begun direct elections during this time. Oregon pioneered direct election and experimented with different measures over several years until it succeeded in 1907. Soon after, Nebraska followed suit and laid the foundation for other states to adopt measures reflecting the people's will. By 1912, as many as 29 states elected senators either as nominees of their party's primary or in conjunction with a general election.
The 1914 United States Senate election in Ohio was held on November 3, 1914. Republican nominee Warren G. Harding, future President of the United States, defeated Democratic nominee Timothy S. Hogan and Progressive Arthur L. Garford to succeed retiring incumbent Republican senator Theodore E. Burton.
