Final answer:
Calhoun's nullification theory, championed by John Calhoun, allowed states to nullify federal laws deemed unconstitutional, notably seen during the South Carolina Nullification Crisis in the 1830s.
Explanation:
Calhoun's nullification theory, promoted by John Calhoun of South Carolina in the 1830s, asserted that if a state deems a federal law unconstitutional, it can nullify it within its borders. This theory was prominently applied during the South Carolina Nullification Crisis, where South Carolina, under Calhoun's leadership, sought to nullify federal tariffs that were perceived as harmful to the South.
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