I read in a book a while ago that slaveholders in Confederate America weren't necessarily avidly pro-slavery, but viewed slavery as a "necessary evil." At least this was until the North started criticizing them and questioning the moral grounds for slavery, which is when Southerners claimed moral justification for their beliefs. Was the South's wild support of slavery a result of the "backfire effect," (someone's beliefs grow stronger when shown contradictory evidence) or did they really have strong feelings about slavery all along?
Basically the whole of the USA was pro-slavery until the era of the American Revolution, with slavery existing in every state though in lesser numbers in the North. Revolutionary rhetoric and the actions of African-Americans who issued trenchant critiques of the limits of liberty if it excluded them led Northern states to begin emancipation and Americans of that generation to find fault with slavery. Southern economies were slave dependant at that point and the necessary evil rhetoric emerged to justify continued slaveholding in an era where it was generally seen as contradicting American ideals. As the 19th century went on, Cotton production emerged, slavery became more profitable and more entrenched, leading slaveholders to feel more secure in their slaveholding. At the same time, abolitionists attacks played a role in heightening Southern rhetoric defending slavery, which began to stress the biblical, natural, and beneficial aspects of the peculiar institution. But yeah, I'd say its both things, the change in slavery's profitability and acceptability and the birth of abolitionism that lead to the change in sentiment in the South.