Republicans began referring to the anti-war Democrats as 'Copperheads,' after the poisonous snake Agkistrodon contortrix, also known as a copperhead. Those Democrats welcomed the moniker, rewriting the copper 'head' as a portrait of Liberty, which they proudly wore as badges cut from Liberty Head big cent pieces. War Democrats, on the other hand, were Democratic supporters of the war. Two Democratic Congressmen from Ohio, Clement L. Vallandigham, and Alexander Long were notable Copperheads. In a sequence of testing in 1864, Republican prosecutors charged certain famous Copperheads of treason.

Copperheadism was a polarising grassroots movement. The territory directly north of the Ohio River, as well as various urban ethnic districts, were its strongholds. According to historians such as Wood Gray, Jennifer Weber, and Kenneth Stampp, it represented a traditionalist element concerned about the Republican Party's fast modernization of society, and it turned to Jacksonian democracy for inspiration. Other historians contend that the Copperheads harmed the Union war effort by rejecting conscription, promoting desertion, and organizing conspiracies, whereas Weber claims that the draught was already in disgrace and that the Republicans vastly inflated the conspiracies for partisan motives.

According to historians such as Gray and Weber, the Copperheads were rigidly anchored in the past and naive about the Confederates' determination to return to the Union. They were obstructionists and partisans who believed the Republicans were demolishing the ancient past they cherished. In turn, the Copperheads were employed by the National Union Party to disparage the leading Democratic candidates in the 1864 presidential election. Copperhead support grew when Union soldiers performed badly and dwindled when they triumphed. After the fall of Atlanta in September 1864, Union military victory appeared to be guaranteed, and Copperheadism began to crumble.