Answer:
Two important political issues occurred in the first half of the nineteenth century caused political alignment along the border lines, reinforcing the identities of the North and South as distinct regions with clear strongly opposed interests that fueled the arguments on the rights of states that culminated in secession and the Civil War. One of these issues concerned the protection rates decreed to help the growth of the industrial sector, mainly in the North. In 1832, as opposed to federal legislation that increased rates, South Carolina passed a nullification ordinance, a procedure by which a state voids a Federal law. Shortly after a naval flotilla was sent to the port of Charleston, and the threat of land troops that landed was used to impose the charging of fees. A compromise was reached whereby the rates would be gradually reduced, but the underlying argument about the rights of the states continued to increase in the following decades.
The second issue concerned slavery, mainly the question of whether slavery would be allowed in newly admitted states. The issue was treated delicately by political commitments designed to balance the number of "free" and "slave" states. However, the issue arose again in a more virulent way, around the time of the US Intervention in Mexico, which increased the interests at stake by adding new territories to the South.