Which political leader is known for opposing tariffs as unconstitutional and emphasizing states' rights?

John C. Calhoun is recognized for his strong opposition to tariffs, particularly during the Tariff of 1828, and for promoting the doctrine of states' rights. He argued that states had the authority to nullify federal laws that they deemed unconstitutional, which was a significant aspect of his political philosophy. This ideology was rooted in the belief that the federal government should have limited powers and that states should retain the right to govern themselves, especially concerning economic interests that did not align with federal policies.

Calhoun's position was reflective of the growing sectional tensions in the United States leading up to the Civil War. He felt that tariffs primarily benefited industrial Northern states at the expense of the agrarian South, where economies were heavily reliant on exports. His advocacy for states' rights and nullification directly challenged the federal government's authority, which contributed to ongoing debates about the balance of power between state and federal entities.

By focusing on the concepts of states’ rights and unconstitutional tariffs, Calhoun's legacy as a political leader is marked by his defense of Southern interests and his opposition to federal overreach. This makes him the correct answer in the context of the question regarding a political leader who most notably supported these ideas.