Senate seating chart, Congressional Directory, 37th Congress, 3rd session, 1863
During the Civil War, the Senate lost 25 members by withdrawal, expulsion, or failure to appear; the House lost more than 60. This 1863 Senate chart shows 49 seats occupied, including one by a Unionist replacement from Virginia. Over time, numbers varied with shifting allegiances of Virginia, Tennessee, Kentucky, and Missouri and the addition of two new states.
Records of the U.S. Senate, National Archives and Records Administration
Congress, Secession, and the Confederacy
With the secession of eleven Southern states in 1860 and 1861, the House and Senate lost more than 85 members. Congress quickly acted to ensure that it could continue to fulfill its legislative responsibilities to the Union. One of the first issues members addressed was whether to recognize secession and withdrawal from Congress as legitimate under the Constitution, or simply to declare the Southern congressional seats vacant. Meanwhile, Southerners who departed drew on their experience in Congress to establish a government for the Confederacy.