The 1866–67 United States Senate elections 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 chosen by state legislatures. Senators were elected over a wide range of time throughout 1866 and 1867, and a seat may have been filled months late or remained vacant due to legislative deadlock. In these elections, terms were up for the senators in Class 3.

The 1864–65 United States Senate elections were held on various dates in various states. They occurred during the American Civil War and Abraham Lincoln's re-election. As these U.S. Senate elections were prior to the ratification of the Seventeenth Amendment in 1913, senators were chosen by state legislatures. Senators were elected over a wide range of time throughout 1864 and 1865, and a seat may have been filled months late or remained vacant due to legislative deadlock. In these elections, terms were up for the senators in Class 2.