Not just defended, expanded. Of course, the dominant southern ideology of the 1850s saw no distinction between the two.
One of the areas of agreement between Fire-Eaters and moderate non-abolitionist Republicans like Lincoln was that both sides believed the institution was like a shark. It had to keep moving ahead or die.
AFAIK, this unproven assumption was never really challenged. The Democratic Party split, ending any hope of stopping the Republicans, over northern Democrats refusal to bow to southern demands they insert a pledge to abandon popular sovereignty in the territories and impose it on the inhabitants whether they wanted it or not via a Federal Slave Code enforced by federal troops. (It is interesting that the last national institution, the Democratic Party, split over southern demands that the power of the Federal Government be expanded.)
Whether slavery had to expand or die was true or not, all southern leaders certainly believed it. Which led to some obvious difficulties for them. Expansion into the remaining territories was a symbolic issue, as none had the climate that would allow slavery to thrive, at least slavery of the type southerners were familiar with.
So we are supposed to assume that southerners, had they been allowed to depart peacefully in 1860/61, would have been content to remain in their existing boundaries, and peacefully watch their beloved institution gradually wither and die.
They wouldn't, of course. They would have promptly attempted to move South and conquer new territory in Latin America.
This would most certainly not have worked, since the only way to attack these areas, given the tech of the time, was via the sea. And the Royal Navy (not to mention the US Navy) would never have allowed conquests in the interests of slavery. That such expansion was possible was a peculiar delusion widely shared in the 1850s South.
Wasn't their concern that if free states continued to be added to the United States, but slave states were not, then it was only a matter of time until the free states had the legislative numbers in Congress to outlaw slavery?