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, 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.

The elections took place amidst rising sectional tension over slavery and the related issue of territorial expansion. In the previous Congress, the debate on the proposed Lecompton Constitution and the application of the Dred Scott decision split the Democratic Party between allies of the senator from Illinois Stephen A. Douglas, who opposed the measures, and the administration of James Buchanan. Douglas was nominated for president by his supporters in 1860, while the Buchanan wing consisting of most Southern Democrats and doughfaces coalesced behind the outgoing vice president and senator-elect from Kentucky John C. Breckinridge. In California, Missouri, and Oregon, vote-splitting between Douglas and Breckinridge Democrats resulted in gridlock that persisted over multiple rounds of balloting. Republicans were the beneficiaries of Democratic infighting, picking up an open seat in Oregon and using their influence to elect a Douglas Democrat over his Breckinridge Democratic opponent in California.

In the slave states, the opposition to the Democrats was fragmented and generally lacked sufficient numbers to threaten Democratic senators. Of the five slave states to hold regularly scheduled elections, only in Missouri was there a protracted struggle over the selection of the state's senator. In Maryland the American Party was the second party in the legislature, and in North Carolina the Whig label was revived; elsewhere the opponents of the Democrats were called Oppositionists or Constitutional Unionists. These groups subsequently would join forces with unionist Democrats and Republicans in Union coalitions that resisted secession in the Upper South and border states during and after 1861.

Besides Oregon, the Republicans flipped Democratic-held seats in Indiana, Ohio, and Pennsylvania and added two seats representing the new state of Kansas, swelling their caucus to 31 senators. The Democrats flipped an American-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; Douglas, who died on June 3, 1861, and was replaced by a Republican appointee; Breckinridge, who was expelled from the Senate in December 1861 after enlisting in the Confederate States Army; and Joseph Lane of Oregon, the former vice presidential candidate of the Breckinridge Democrats, whose seat was filled by a Douglas Democrat.