It was aspirational identification.
Even though the great majority of southern whites had essentially no prospect of rising to the wealth and status of the planter families, they were the ideal to which the rest of the workers in the economy aspired.
That’s why the spread of slavery was such an incendiary one after the Mexican-American War: working whites couldn’t hope to displace the entrenched planter families—they owned the good land in the existing slave states—the aspirations of the lower classes required the spread of slavery for the model to be sustained.
Freeholding northern whites, in turn, understood that this expansionist imperative was a direct threat to their aspirations, and they formed (with railroad financial support) the core supporters of the nascent Republican Party.
Excellent post. You hit the nail on the head. The reasoning above explains virtually all the facts surrounding the politics of secession.