Before the start of the Civil War, many Southern political leaders supported State rights.
The Southern states had many differences with the Northern states and the US government. The Southern states wanted more liberties and rights because they thought their situation was different and their economy depended so much on slavery to work in large plantations to produce the cash crops needed to trade and export to England. From that idea of more State rights, some states such as South Carolina wanted to secede from the Union.
A two-thirds majority vote in both the House of Representatives and in the Senate is required to override a presidential veto. The exact number depends on how many representatives vote; therefore, the actual number is subject to chance