The North and the South disagreed heavily on the topic of slavery. The North wanted it destroyed, but the South wanted it to spread. As time passed the tension built. One of the tipping points was when John Brown tried to start a revolution by stealing guns for the slaves from a weapons depot in the South. He was caught before anyone was hurt and sentenced to hang. The North call him a martyr, which enraged the South, since he had been planning to hurt and kill them.
Most of the time when slavery was brought up in the time before the civil war at Congress, it would either be ignored or shut down because congressmen were divided on the issue. When Lincoln ran for president, he openly discussed his wanting to end slavery which led to southern states saying they would split if he was elected. Lo and behold, he was elected so the southern states split off into the Confederacy starting the Civil War. This is probably a small part of how the politics of slavery helped to ignite the Civil War, but it gives you an idea on how divisive slavery was among politicians and the states.
