Slavery was highly contentious, at all levels, from day 0 of the United States. That's where Thomas Jefferson's "All men are created equal." line came from. In the original draft of the Declaration, he included a lengthy diatribe against slavery, but it was removed in the final draft - presumably under pressure from slave interests. That conflict, over such a meaningful issue, still took near to 100 years to collapse into the Civil War.

Back to modern times, I don't think it's hyperbolic to see secessionary momentum following the election next year, regardless of who wins, as an at least reasonably possible scenario. And if it does, then we're 9/10ths of the way there. And it's not even entirely clear what happened. It's just like things divided hard in an incredibly brief period of time.