The 1966 United States Midterm Elections were held on November 8, 1966, during the presidency of Lyndon B. Johnson. These elections were significant as they determined the composition of the 89th United States Congress, including all 435 seats in the House of Representatives and 34 of the 100 seats in the Senate. The elections resulted in a gain for the Republican Party, which won 47 seats in the House and 2 seats in the Senate. This shift reflected growing public discontent with the Democratic Party and the ongoing Vietnam War, impacting the political landscape leading into the late 1960s.