Copperheads "Peace Democrats"
Republican Abraham Lincoln was able to win the 1860 presidential election largely because the Democratic party had torn itself into several factions and could offer no united opposition. In the North the Democrats divided into two factions- the War Democrats and the Peace Democrats. Neither group agreed with the way the Republican administration conducted the war, but the War Democrats at least supported the fight for the Union.
The Peace Democrats were opposed to the war and would have accepted a negotiated peace resulting in an independent Confederacy. Most Peace Democrats were from the midwestern states of Ohio, Illinois, and Indiana, but political dissent was widespread throughout the North. Midwesterners had close economic and sentimental ties with the South, and many of them bitterly opposed the Union's war against what one of them called "the injured, incensed, downtrodden people of the South."
In 1861, Republicans started calling antiwar Democrats "copperheads", likening them to the poisonous snake. By 1863, the Peace Democrats had accepted the label, but for them the copper "head" was the likeness of Liberty on the copper penny, and they proudly wore pennies as badges.
The Copperheads mounted a forceful and sustained protest against the Lincoln administration's policies and conduct. The most popular of the Copperheads was Democratic Congressman Clement L. Vallandigham, who in 1862 introduced a bill in Congress to imprison the President. Instead, Vallandigham and a host of other Democrats, including judges, newspaper editors, politicians, and antiwar activists, were arrested and imprisoned without trial on the orders of Lincoln and Secretary of War Stanton, who had decided to take off their gloves in dealing with persons "guilty of any disloyal practice".
Fascinating Fact: At the 1864 Democratic convention, Vallandigham persuaded the party to adopt a platform that declared the war a failure and called for negotiations with the Confederacy.
You've hit it. This is exactly what I proposed on another thread. Lincoln suspended the writ of habeus corpus and did them one better by arresting them and holding them without charges. It took quite a while for the courts to get to the point where Lincoln decided to free them and a number of them left the country for the duration of the war. That would be a real plus now days.