Copperheads, in the American Civil War, a reproachful term for those Northerners sympathetic to the South, mostly Democrats outspoken in their opposition to the Lincoln administration. They were especially strong in Illinois, Indiana, and Ohio, where Clement L. Vallandigham was their leader. The Knights of the Golden Circle was a Copperhead secret society. The term was often applied indiscriminately to all Democrats who opposed the administration. It afforded an opportunity for impugning the loyalty of those who opposed Lincoln's policies, either military or civil (e.g., the suspension of habeas corpus), and it was not until years after the Civil War that the Democratic party succeeded in living down the association.
See W. Gray, The Hidden Civil War (1942); F. L. Klement, The Copperheads in the Middle West (1960).
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.