Did I read it was 1/3 of the south had slave?
More like 5%, but that five generally had family members, so certainly a number of times that 5% had an interest in preserving slavery.
Leep, here's how the 1/3 of the south had slaves numbers work.
A) In Alabama it's somewhere between 30% to 40% of white families had slaves (so yes, about 1/3).
B) A family is counted as owning the slave even if only one person did (i.e. the husband/father). I guess that's fair, like today saying our family owns our house even if it's in only my name and not my spouse's too and certainly not owned in my now grown kids' names.
C) A family is counted as owning slaves in all generations even if the family tried it for just a few years and decided it wasn't their cup of tea and freed the slaves (or their fortunes changed and they had to sell the slaves). This happened a lot. My family in Alabama never owned slaves (my mother's the descendent of sharecroppers from another state and my father is from a northern part of Alabama where slavery was practically non-existent even in farming communities like the farm my father grew up in). But let's pretend I had one ancestor who owned a slave or two for about 3 or 4 years and decided he didn't like it and freed the slave, and that was the only slave anybody in my family ever owned. That one event by one person would make my family counted as a "white family that owned slaves" even if almost the entirety of my family's existence we didn't own slaves nor wanted to. It'd be analogous to defining a family today as a "bank robbing family" if only one person in the family robbed a bank in the 1830's and it was only once in his life.
So think about that from now on when you point out or hear that 1/3 of white southern families had slaves. That's accurate, but within context that's not like what it sounds.
While it’s possible that 30% of the Southern population were slaves, that is not the same as saying that 30% of Southerners owned slaves.