Hiya!

/u/Georgy_K_Zhukov addresses your question excellently in his superb write-up on /r/badhistory discussing the causes of the Civil War and the motivation for the Confederacy's secession - slavery.

The short answer to your question is that the secessionists had nothing to fight when they seceded, but that they were acting in anticipation of a long-term erosion of their congressional position. Lincoln and the Republicans made no direct moves in Congress towards abolition. Rather, the confederates were worried that their position, as you suggest, was unentenable in the long term, and in recognition of the fact that they would not be able to legally defend slavery forever, they moved to act pre-emptively. Although I would -super- heavily recommend reading Georgy's whole write-up because his prose are as enchanting excellent as his smooth, silky voice his argument, the most relevent section is here:

"When Lincoln was elected in the fall of 1860, the South was terrified. Whatever his prior declarations that whether he wished to or not, he had no power to interfere with the institution where it existed, Lincoln was nevertheless a Republican, a political party founded on its opposition to slavery, and at its most mild, committed to stemming the further spread as statehood spread westward. While committed, absolute abolitionism was a vocal minority on the national stage, the simple limiting of expansion presented a long term existential crisis to the slaveholding states. Every free state to enter the Union represented additional Senators and Representatives to immediately exercise power in Congress, and represented the growth of power not only in future Presidential elections, where anti-slavery parties could continue to gain momentum, but in the long term even foreshadowed, one day, a strong enough majority to abolish the institution once and for all through Constitutional Amendment. And it wasn't only that Lincoln and the speedy rise of the Republican party threatened a political threat to slavery, but also that, due to the 3/5 Compromise, the existence of enslaved populations represented a significant boost to the electoral power of the slave states.^3b"

I hope this helps!

3 Themes of the American Civil War: The War Between the States edited by Susan-Mary Grant and Brian Holden Reid - Southern Secession in 1860–1861 by Bruce Collins

They'd already lost the battle in Congress, effectively, the election of Lincoln was perceived to be their last battlefield. Prior to the civil war there was a growing imbalance between slave states and non-slave states, with the most recent 5 states being added to the union (Oregon, Minnesota, California, Wisconsin, Iowa) being non-slave states. That pattern was set to accelerate over the next several years, with the very strong potential of so many new non-slave states being added that in total they'd have a 2/3 majority in both houses, capable of bending federal laws against the institution of slavery and also making it possible to outlaw slavery through constitutional amendment.

In the presidential election Lincoln was seen by folks in the South as an enemy of slavery and they sought to fight against his election in every way possible. When he won election without even being on the ballot in most southern states the leading politicians in the South correctly perceived that event as the swan song of their political power on the federal level. All of the "wins" for the proponents of Slavery in the years prior to the civil war were tactical (such as the Dred Scott ruling) whereas their major losses were strategic.