1966 United States Senate elections

1966 United States Senate elections

← 1964 November 8, 1966 1968 →
35 of the 100 seats in the United States Senate 51 seats needed for a majority
Majority party Minority party   Leader Mike Mansfield Everett Dirksen Party Democratic Republican Leader since January 3, 1961 January 3, 1959 Leader's seat Montana Illinois Seats before 67 33 Seats after 64 36 Seat change 3 3 Popular vote 12,971,141[1] 13,654,168[1] Percentage 48.1% 50.6% Seats up 20 15 Races won 17 18
Results of the elections:      Democratic hold      Republican gain      Republican hold      No election Rectangular inset (S. C. and Va.): both seats up for election
Majority Leader before election Mike Mansfield Democratic Elected Majority Leader Mike Mansfield Democratic

The 1966 United States Senate elections were elections on November 8, 1966, for the United States Senate which occurred midway through the second (and only full) term of President Lyndon B. Johnson. The 33 seats of Class 2 were contested in regular elections. Special elections were also held to fill vacancies. With divisions in the Democratic base over the Vietnam War, and with the traditional mid-term advantage of the party not holding the presidency, the Republicans took three Democratic seats, thereby breaking Democrats' 2/3rds supermajority. Despite Republican gains, the balance remained overwhelmingly in favor of the Democrats, who retained a 64–36 majority. Democrats were further reduced to 63–37, following the death of Robert F. Kennedy in June 1968.

These were also the first elections held after the enactment of the Voting Rights Act of 1965. Upon Edward Brooke's election in Massachusetts, he became the first African-American U.S. Senator elected after the end of Reconstruction and the first-ever popularly elected, as Reconstruction ended before the passage of the Seventeenth Amendment to the United States Constitution. As of 2024, this is the most recent Senate election cycle in which no House incumbents were elected to the Senate and the last time any state elected different parties to each Senate seat in a double barrel election.