When the Republicans won the election in 1860
(neither GOP or GQP at that point) it was on a platform that respected the right of slavery to exist where it did currently but to expand no further. Nevertheless, slave states began to secede, beginning with SC.
Speaking of the long game. The slaveholding oligarchs had most of their actual net worth wrapped up in their human livestock* and breeding them was actually the most profitable part of the enterprise. But the Delta and TX were getting pretty well settled and they needed more land.
The consequence of not getting it was that the value of their slave output would decline steadily, and actually come to outnumber white folks in a couple generations.
So they acted rationally, even if the portion of the 95% who didn't probably did not. Although many of them were drafted into the war and risked being shot or hanged by home guards if they didn't comply.
*more than half the estimated total capital stock.