What class tensions emerged in the antebellum South?

Antebellum society in the South was rigid and hierarchical. The economy was largely based on agriculture—especially the cultivation of cotton—which meant that the middle-classes were very small in number, certainly by comparison with the industrialized North. Political and economic power was held by a small planter elite, which owned huge tracts of land as well as slaves. The vast majority of whites, however, owned very little land or slaves, especially the dirt-poor sharecroppers who eked out a precarious living on the plantations.
Potentially, Southern society had all the ingredients for serious unrest. Yet what prevented social tensions from coming to the boil was the institution of slavery. Whatever position Southern whites occupied on the social ladder, they were united in their belief in the benefits of the peculiar institution. Rich or poor, whites were bound together by racism, by the implacable belief in white supremacy. No matter how poor a white man was, he still regarded himself as better than any black man, slave or free. For such a man, the persistence of slavery was a confirmation of this.
Moreover, poor whites accepted slavery as they were genuinely concerned that its abolition would lead to their being undercut by black workers and priced out of a job. They also shared the pervasive fear of white plantation owners of an imminent slave uprising, which they believed would result in the indiscriminate slaughter of white folk, irrespective of their social status. For these, and other reasons, slavery helped to defuse class tensions, binding white people together in a social hierarchy which everyone had an interest in maintaining.