When you think the gulf between left and right is insurmountable...
When you think, "How could this country be any more divided?"
Before Abraham Lincoln even took the oath of office in 1861, seven (!) U.S. states seceded from the Union.
That's right, seven Southern states, led by South Carolina (naturally) decided that the prospect of a Lincoln Presidency was so abhorrent that they didn't even give him a chance to govern. They opted out.
What was Lincoln's offense?
Well, he had the audacity to suggest that future U.S. states should be free states, not slave states. This did not sit well in the Deep South.
Lincoln took the oath of office, and the South decided to fire the first shots at Fort Sumter, leading to a senseless, bloody war no one wanted, but one which Southern apologists continue to reframe as a war about "state's rights," not slavery.
Delete slavery from the equation, and there is no Civil War. If you don't believe me, read this speech by the Vice President of the Confederate States of America (and a tiny, weak man, barely five feet tall), Alexander Stephens:
"Cornerstone Speech," Savannah, GA, March 21, 1861.
If you don't have time to read the entire speech, here's a summarizing sentence:
"...the negro is not equal to the white man; ...slavery subordination to the superior race is his natural and normal condition."
Alexander Stephens, VP of the CSA. Too small for a properly-tailored suit, Stephens deplored the inferior status of "the Negro."
Are we then to be surprised when a white politician from South Carolina shouts "You lie!" during a State of the Union address delivered by the first Black President of the United States?