Answer: The issue of slavery.
The North and South were strongly divided during the election of 1860 between Abraham Lincoln (Republican Party), Stephen Douglas (Northern Democrat), John C. Breckenridge (Southern Democrat) and John C. Bell (Constitutional Union Party).
The Northern states were in favor of ending slavery because they were Industrial states, the majority of the population worked on factories and lived in big cities and slavery had died in those states. In the South agriculture was predominant they focused on large-scale farms and crops of tobacco and cotton,and they didn't want slavery to end.