"On January 21, 1861, in a scene of high drama, Mississippi Senator Jefferson Davis and four southern colleagues rose on the Senate floor to offer final remarks before withdrawing from the body and returning home to their states. Officials from the South had long supported states’ rights as protection against the central government. During the previous decades, southern senators had frequently used the threat of secession to encourage northern states to compromise on such issues as the spread of slavery into new territories. Believing that the election of Abraham Lincoln to the presidency threatened the institution of slavery and that there was no longer any hope of compromise, southern states began to act on the principle they had so often supported— that states were sovereign bodies that had a right to withdraw from the Union or nullify offensive acts of Congress.
Southerners were adamant about preserving their way of life and economy. Prior to the Civil War, southern states were the world’s largest supplier of cotton. Northern industries purchased the raw cotton and turned it into finished goods like clothing. Slaves provided the labor that made the South’s economy rich in the production of cotton.
In November 1860, a deeply divided nation teetered on the brink of a civil war. In December 1860, South Carolina declared independence from the United States. Immediately, the Senate established a committee to examine plans to save the Union, including a proposed compromise that would have extended the 1820 Missouri Compromise line to the Pacific. With the exception of Missouri, slavery was prohibited north of this line that followed the 36th parallel.
In January 1861, Mississippi became the second state to secede. Following his state’s secession, Senator Jefferson Davis addressed the Senate, imploring his colleagues to allow for peaceful secession of the southern states. Soon five senators from Florida, Alabama and Mississippi, led by Davis, left the Senate. The secession of southern states and the withdrawal of their elected representatives forced an unprecedented constitutional crisis in Congress.
In March 1861, the Senate debated the question; did the states or their senators have the right to leave the Union? Some senators insisted that the southern states did not have the right to withdraw from the Union. By leaving, the southern senators had effectively resigned their seats. Others believed states did have the right to secede and that seats held by southern secessionists no longer existed. Therefore, the Senate should not declare their seats vacant but simply strike their names from the roll.
After a heated exchange, the Senate passed a resolution declaring the seats of their departed colleagues “vacant” and authorized the Secretary of the Senate to strike their names from the Senate roll. After the war began and hope of reconciliation faded, ten Senators were expelled in July 1861 for disloyalty to the Union. That number reached fourteen on by February 1862 when Jesse Bright of Indiana became the last senator expelled for supporting the Confederacy.
A few states, such as Missouri and Kentucky, elected new members to replace those who were expelled. The Unionist government in Virginia sent two senators to Capitol Hill. Many desks remained unoccupied in the Senate Chamber throughout the war years and into the Reconstruction era, serving as painful reminders of the nation’s disunion. The Senate continued to admit southern members from reconstructed states to representation through the early 1870s."
Source: U.S. Senate webpage http://www.senate.gov/artandhistory/history/common/expulsion_cases/CivilWar_Expulsion.htm