Why did southern states attempt to secede from the Union during the Sectional Crisis?

The states
of the Deep South (South Carolina, Alabama, Florida, Mississippi, Louisiana, Georgia, and Texas)
left the Union in the months that followed the election of Republican Abraham Lincoln to the
Presidency. They did so, simply stated, because they saw Lincoln's election as a direct threat
to the institution of slavery. This position was articulated most clearly in the secession
ordinance of Mississippi, which read in part, and is worth quoting at length:

Our position is thoroughly identified with the institution of
slavery-- the greatest material interest of the world. Its labor supplies the product which
constitutes by far the largest and most important portions of commerce of the earth...[A] blow
at slavery is a blow at commerce and civilization. That blow has been long aimed at the
institution, and was at the point of reaching its consummation. There was no choice left us but
submission to the mandates of abolition, or a dissolution of the Union, whose principles had
been subverted to work out our ruin.

Southerners would
later attempt to downplay the centrality of slavery to secession, but their statements at the
time of secession made it clear. Though Republicans in general, and Lincoln in particular,
advocated only a policy of restraining--not abolishing--slavery, many Southern politicians
argued that the spirit of abolition was so strong in the North that it would only be a matter of
time until a majority of Northerners supported the destruction of slavery. They regarded even
the territorial restraint of slavery as an existential threat to the institution, and feared
that Lincoln would use his powers of appointment to fill federal positions--including customs
collectors and postmasters--with antislavery men. Other secession ordinances, including South
Carolina's, pointed to Northern states that had adopted personal liberty laws in defiance of the
Fugitive Slave Act.

Some southern politicians had long cited a
"compact" theory of the Constitution that made, they argued, secession legal. They had
used the threat of secession for many years to gain political concessions that favored slavery.
After Lincoln's election, seven of the slave states put this theory into action because they
believed they could no longer expect a federal government that actively supported slavery. Four
other slave states--Virginia, Tennessee, Arkansas, and North Carolina--did not leave the Union
until Lincoln's call for volunteers in the aftermath of Fort Sumter in April of 1861. Maryland,
Delaware, Kentucky, and Missouri, all slave states, remained in the Union.

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