I’ll let Robert Toombs answer that question. He was a senator from Georgia, a leading spokesman for secession, later Secretary of State for the rebel government, and a general in the rebel army. You’ll notice this letter was written nine months before the 1860 election.
“In view of such effects and consequences here from the mere possession of one branch of Congress we ought not to shut our eyes to the effects of the possession of the government in all of its departments by any Black Republican. It would abolitionize Maryland in a year, raise a powerful abolition party in Va., Kentucky, and Missouri in two years, and foster and rear up a free labour party in [the] whole South in four years. Thus the strife will be transferred from the North to our own friends. Then security and peace in our borders is gone forever. Therefore I deeply lament that any portion of our people shall hug to their bosoms the delusive idea that we should wait for some “overt act.” I shall consider our ruin already accomplished when we submit to a party whose every principle, whose daily declarations and acts are an open proclamation of war against us, and the insidious effects of whose policy I see around me every day. For one I would raise an insurrection, if I could not carry a revolution, to save my countrymen, and endeavor to save them in spite of themselves.”
Letter from Senator Robert Toombs to Alexander Stephens-February 10,1860
My point is that slavery was not threatened, and the Union would continue practicing legal slavery for decades more if the South had remained.