Final answer:
John C. Calhoun was a key figure in defending southern interests, including slavery, by promoting states' rights and the concept of a concurrent majority, and by taking steps such as supporting nullification of federal tariffs.
Step-by-step explanation:
John C. Calhoun was a staunch defender of southern policies and slavery. He deemed that the North was responsible for the crisis the Union was facing, due to their restrictive stance on slavery and the admission of new territories from the South. Calhoun's actions included advocating for the South to have an equal right in the acquired territory, ensuring the North fulfilled its duty regarding fugitive slaves, and potentially adding a constitutional provision that would restore southern power lost due to an imbalance caused by government action.
Calhoun advanced the idea of a concurrent majority, which would allow the minority Southern states to veto legislation by the majority, in an effort to protect slavery and Southern rights. He also openly participated in the nullification crisis, opposing the Tariffs of 1828 and 1832 by supporting a state convention to nullify these acts. Calhoun's actions and rhetoric greatly contributed to the growing sectional divide, eventually leading the Southern states towards secession and the Civil War.