Slavery, like it or not, was constitutional and had been protected by the courts as such for nearly 100 years. Had slavery been an issue, the southern states would have had recourse through the existing justice system.
The power of taxation, which Lincoln had made clear would be used to subjugate southern agrarian interests to those of northern manufacturing, was a constitutionally delegated power of the national government.
Avoiding abuse of that power required secession.
So you're suggesting that slavery was not a reason for the southern rebellion?
It's not as simple as that. The question wasn't whether or not slavery was constitutional. The question was to what degree did Congress have the power to regulate or prohibit its expansion. Going back to Jefferson's ordinance of 87, the precedent had been set that Congress could and perhaps should prevent the expansion of slavery into new territories. This was the approach taken in the Wilmot Proviso, which I am sure you are familiar with. Well, the slave states damn near had a cow at the thought! It was at the time of the Wilmot Proviso, all the way back in the early 40s, as I recall, that the Slave states first started making noise about secession. The issue was slavery. It didn't matter to them if Congress had the authority or not, it was a line in the sand for them.
You would think that the question of Congressional authority to regulate slavery in the District of Columbia would not have been as controversial, but it was.
Article 1, Section 8 gives Congress the power "To exercise exclusive legislation in all cases whatsoever, over such District (not exceeding ten miles square) as may, by cession of particular states, and the acceptance of Congress, become the seat of the government of the United States..."
However, when Congressmen such as John Quincy Adams attempted to exercise that authority to ban slave auctions and the practice of slavery, hell, they couldn't even bring the bills to the floor. The House version of the Calhounists routinely walked out. Not exactly republican behavior. For them, the mere thought of any limit on their precious trade was reason for anarchy. What else can you call it when they walk out of Congress when they don't get their way? Or when debate on something they don't like is attempted?
In reality, as Americans migrated west, as the frontier was tamed, as new states were formed, the question of slavery--and it was a question--was a crippling impediment to progress. It held up California statehood (the slavers wouldn't accept them because they were to be--by their own choosing--a free state. So much for states' rights!)It held up the building of the railroads.
Now, if it were me, I would support the authority of Congress, per section 8, to prohibit slavery in new territories, AND in D.C. To me, that's a plain reading of the law. But if it were my line in the sand, an issue so near and dear to me that I would die for it, that I would turn on my own nation's flag, well then, I don't suppose Article 1, Section 8 counts for much. That's anarchy, my friend.