The key political figure in the American South advocating for states' rights pre-Civil War was John C. Calhoun. He championed the idea that individual states had the right to govern themselves and resist federal authority when it conflicted with their interests. Calhoun's position reflected Southern concerns about federal intervention, particularly regarding slavery and economic policies. His advocacy for nullification, the theory that states could invalidate federal laws, contributed to the rising tensions that eventually led to the Civil War.