IOW: Northern pro-slavery Democrats.
That conclusion isn’t logical or accurate historically. Economics, not slavery, led to the war. in 1861 Lincoln would have helped cement slavery into the constitution to prevent secession. The US couldn’t afford to lose access to the Mississippi river and the excise taxes that flowed from southern ports: Charleston, New Orleans and Galveston. Slavery was a state issue, not a federal one. Copperhead was a perjorative label applied to politicians who were strict constructionists of the Constitution. Almost all hated the institution of slavery but believed the federal govenment had exceeded its powers by making war on states optin to leave the union they had entered freely.