In the 1850s, not unlike today, America was two nations. Then it was half free and half slave. When we look to make the comparison to today, we often miss the point. It's not about the comparisons to the Civil War itself, but to the events, the efforts, the policies that actually led to the war. In fact, the war before the war.
That period is examined closely by Andrew Delbanco in The War Before the War: Fugitive Slaves and the Struggle for America's Soul from the Revolution to the Civil War.
My conversation with Andrew Delbanco: