I can't see the boxes I'm supposed to check, but I'll give an answer: slavery.
Explanation:
The Republicans were an anti-slavery party, meaning they wanted to stop the expansion of slave states or abolish it entirely. The Dixiecrats were terrified of abolition of slavery, and fought as hard as they could to prevent it. When Lincoln, a Republican, was elected without even winning the south, it caused them to react with succession to protect the institution of slavery.