Southern leaders feared that the election of Abraham Lincoln representing a turning point in American politics portending the end of slavery and the forced intermingling of races in the South. We know this because those tasked with explaining secession and convincing other Southern states of its merits stated as much. The Alabama declaration of the causes of secession explained that Lincoln's government "advocates negro equality, socially and politically, and promotes insurrection and incendiarism in our midst," which would eventually ruin "the sacred purity of our daughters." Likewise, a commissioner from Alabama tasked with spreading secession throughout the South warned others that the election of Lincoln was "nothing less than an open declaration of war, for the triumph of this new theory of government destroys the property of the South, lays waste her fields, and inaugurates all the horrors of a San Domingo servile insurrection, consigning her citizens to assassinations and her wives and daughters to pollution and violation to gratify the lust of half-civilized Africans."

Sometimes the issue of slavery can be confused because Lincoln announced his support for the Corwin amendment, which had passed Congress and if ratified by the states would have prevented the Federal government from interfering with the institution of slavery. But Lincoln was a staunch abolitionist, and his rise to the presidency was understood by Southerners as the culmination of a much longer battle between the North and the South over the future of slavery. With Lincoln in power, the writing was on the wall for the South: no more slave states would be admitted into the Union, and thus power would shift in the coming decades to the already-powerful North, giving it a preponderance of political power which it could and would end slavery in the United States. Nothing short of complete secession, Southerns believed, could stop the new party of Lincoln from destroying slavery and transforming the South into a multi-racial wasteland.

The quotes I cite can all be found in Charles Dew's Apostles of Disunion: Southern Secession Commissioners and the Causes of the Civil War, Charlottesville: University Press of Virginia, 2001, a relatively short work written for a Southern audience that may believe that "states' rights" played a key role in secession.