So the Constitution did protect slavery. The Constitution states that, “Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons.” Article 2
The Constitution clearly meant to allow for slavery to exist. And of course, numerous laws going forward would confirm this, from the Fugitive Slave Law to the Missouri Compromise to the Dred Scott Decision. Now, the question of whether the slave-holding states had a right to secede is a tricky one. The Constitution is unclear about that. The chief architect of the idea of secession in many ways is John C Calhoun. In 1828 Calhoun published the South Carolina Exposition and Protest. It made an argument for nullification, that is, a state’s right to void a Federal Law. Essentially, this meant that the federal government could force the issue and make the law constitutional, or not. If it did make the law constitutional, then the opposing state could either secede or not. Calhoun believed it was better to part ways at this point in his life than side with the North on issues like tariffs, which hurt the South but helped the North (American Lion, John Meacham, 58).
Andrew Jackson believed that nullification and secession went hand in hand, and also stated that it was “incompatible with the existence of the Union, contradicted expressly by the letter of the Constitution, unauthorized by its spirit, inconsistent with every principle on which it was founded, and destructive of the great object for which it was formed.” (American Lion, Meacham, 226) In essence, Jackson, believed that the Constitution formed a government and not alliance of some sort (American Lion, Meacham, 227). This is the crux of any argument on whether it was legal for the states to secede in 1861.
Lincoln put it best. He believed the argument over secession was very simple. He saw that what was really at stake was the question of whether a government, peacefully and freely elected (that is to say, the president’s administration) could be undermined and abandoned just because the party-in-exile didn’t like that it had lost (1861, Adam Goodheart, 320). Consider how many presidents had been pro-slavery or slave-owners over the first 70 years of the Union. Yet the North never left. So Lincoln’s point, although a philosophical one, is also a legal one. How could a Constitution voted on by the states and approved by all the states be so easily abandoned, when the states knew the rules of the game? Yes, the constitution clearly protected slavery; that wasn’t in question. But if the North were able to get a 2/3s vote banning slavery put into the Constitution (this was a possibility, as the North was starting to surpass the South in power)? Even then the South could not secede; the states had approved the Constitution in its entirety.
This is why Lincoln did not acknowledge the Confederacy. Because to him, the Confederacy couldn’t exist legally; the southern states were in rebellion, but never had they legally established their own government. So to answer your question, in my opinion, many southern leaders and states tried to justify their actions on legalese foundations, but in the end, the Constitution simply did not grant them that right. They had entered willingly into a contract and constitution; they couldn’t simply leave. In fact, in my opinion (and this is just my opinion) when the Founders wrote the “necessary and proper” clause of the Constitution, it is my belief that at least some had harbored secret hopes that down the road, the Federal government would use this power to eliminate slavery, when the nation was prepared for such an action. I think we see that moment in Lincoln’s Emancipation Proclamation and of course, in the overturning of slavery by the Congress in 1865.
I hope that helps.
I am just chiming in to show you this excellent response by /u/Borimi to this question in an earlier thread.
Hope this helps!
Article 6, Section 2 of the Constitution states: "This Constitution, and the Laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land; and the judges in every state shall be bound thereby, anything in the constitution or laws of any state to the contrary notwithstanding."
Doctrines of nullification and threats of succession aside, I doubt anyone seriously thought that the Federal Government would stand pat while states attempted to secede, be it New England via the Hartford Convention in 1812, or Southern states. Political threats of this nature were as much grandstanding and a test of will as anything else.
Had the South legitimately thought secession was legal, they might have made some attempt to try it via court. They didn't - they immediately prepared for war as soon as they announced their intention to secede. It's not like they had to worry about a Supreme Court packed against them - Dred Scott was decided 7-2 4 years prior.