Why did president Lincoln believe the copperheads were a threat to the union war effort?

The Copperheads were a group of Northern Democrats who opposed the American Civil War, wanting an immediate peace settlement with the Confederates. They were also called Peace Democrats, because they were in favor of a compromise peace.

The Confederates believed that if they could prolong the war, and cause enough war-weariness in the North, Lincoln could lose the 1864 election. If so, the South might gain its independence.

Unfortunately for the Copperheads, the fall of Atlanta in September 1864 meant that the Radical Republicans prevailed, and Lincoln was re-elected. By the time of his assassination on April 15, 1865, the war was essentially over.