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.
Its peculiar you of all people would attempt to draw a link between Saddam Hussein's political operation and the confederacy. After all, are you not the same individual who practically went off a very verbiose deep end when I simply pointed out Karl Marx's embrace of The Lincoln's agenda? You blasted me with complaints that I dare draw any relation between the Marxist movement and The Lincoln, even though historically there was a shared political position between them. Yet here you are using language that connects the confederacy to a completely unrelated islamo-arab political movement from 130 years later and on the other side of the globe. What gives?