1860–61 United States Senate elections

The 1860–61 United States Senate elections were held from January 14, 1860, to April 2, 1861. Regularly scheduled elections were held for 23 out of the 68 seats in the United States Senate, and special elections were held in California, Oregon, Maine, Pennsylvania, and Ohio. One seat was previously elected on December 12, 1859. Following the start of the 37th Congress on July 4, 1861, special elections were held in Virginia and Kentucky to fill vacancies resulting from the secession of the Confederacy. The Republican Party flipped six Democratic-held seats and gained control of the Senate for the first time following the departure of senators representing Confederate states.

U.S. senators are divided into three classes whose six-year terms are staggered, such that one-third of the Senate is elected every two years. Senators in Class 3 were elected in 1860 and 1861. Prior to ratification of the Seventeenth Amendment in 1913, senators were elected by the U.S. state legislatures. There was no fixed calendar, and states held elections on various dates preceding the first session of Congress. In states with split partisan control of the legislature, multiple rounds of voting could be required to elect a senator, leading to extended vacancies.

Republicans flipped Democratic-held seats in Indiana, Ohio, Oregon, and Pennsylvania and added two seats representing the new state of Kansas, swelling their caucus to 31 senators. The Democrats flipped a Know Nothing-held seat in Kentucky, but lost five seats from states that seceded before the end of the 36th Congress; the departure of 16 more Southern senators before July 4 reduced the Democratic caucus to 15 seats. In special elections held after the start of Congress, Unionists filled two vacancies in Virginia and flipped a Democratic-held seat in Kentucky, reducing the Democratic caucus to 14 seats by the end of 1861.

The Republican victory in the 1860 United States presidential election precipitated the resignations of senators Hannibal Hamlin of Maine, Simon Cameron of Pennsylvania, and Salmon P. Chase of Ohio, who became vice president, secretary of war, and secretary of the treasury, respectively, in the incoming Lincoln administration; Republicans won all three special elections to select their replacements. Other notable departures included William H. Seward of New York, who retired in order to accept an appointment as secretary of state; Stephen A. Douglas, who died on June 3, 1861, and was replaced by a Republican appointee; John C. Breckinridge, who was expelled from the Senate in December 1861 after enlisting in the Confederate States Army; and Joseph Lane of Oregon, whose seat was filled by a Douglas Democrat.