Whether you believe the constitution is a living document or you are a strict constructionist, you have to agree that there's disagreement on what its intent is.
Just like the bible or any other sacrosanct document, people first decide what they believe, then examine the document for how they can prove it validates what they believe.
There's no longer any pretense of nominating supreme court judges based on their credentials or loyal following of the law. It's pretty much a naked tabulation on how closely their rulings can advance the nominating politician's stance on the issues of the day or at the very least, what their party masters want.
Buchanan was no different from politicians in the modern age in wrapping his social and economic views in the constitution.
As I've mentioned before, although he was from Pennsylvania, his sympathies were completely with the southern slave interests.
Somehow, Buchanan saw fit to send federal troops in to capture escaped slaves in Massachusetts, but could not see justification in using the federal army to put down armed rebellion in the south.
As southerners in South Carolina (where else?) were seizing federal military bases, Buchanan would not lift a finger based on his 'constitutional' views.
He argued that no state has the right to secede from the Union, however, all bodies under any Constitution had the right to take up arms to prevent oppression. The federal government he argued had no power to either recognize the secession of states or coerce them through arms. In other words, the Federal government can never tell the states what to do, even if they're threatening to not be part of the government anymore.
Does this make sense? No.
But that's not really the point, then, as now, all low minded arguments are wrapped in the high minded language of our founding fathers.