Slavery was guaranteed by the U.S. Constitution at the time. Why would the South go to war over it? Ya can look it up. PUNISH THIS!
1) Because Southern elites thought the Republican opposition to the expansion of slavery and the Northern opposition to fugitive slave laws unconstitutionally threatened slavery.
2) Because they thought they would fare better in a Ba'athist slaveholders' republic than in a Republican-ruled union.
Southerners would lose out on other issues as well when Republicans dominated (though not as much as one might think, given the Senate rules of the day, which benefited them as members of the minority).
But the Deep South states had already resolved to leave, rather than to fight politically within the union. And if you want to ask why things reached the point they did, you can't ignore the conflict over slavery. If Southern extremists hadn't gone crazy about slavery, the Democrats, who were favorable to Southern and slaveholding interests, would have remained in power, and there would have been no war in 1861.