No he did not - at least not in the parts of his State of the Union speech referenced in post 289. Buchanan makes no constitutional-rooted claim that secession is illegal.
Buchanan instead, invokes a religious test from the Sermon on the Mount: “Surely under these circumstances we ought to be restrained from present action by the precept of Him who spake as man never spoke, that “sufficient unto the day is the evil thereof.””
It is fine that Buchanan used a Biblical reference. Nothing wrong with that.
But I believe if Buchanan had a compelling Constitutional argument that would negate the 9th and 10th amendments, and the founding principal “consent of the governed,” he would have verbalized it in his supposed “unequivocal line against secession in theory.”
As would you.
I don't have time to go through everything Buchanan said or didn't say. But here's one thing he said:
“Such a principle [of secession] is wholly inconsistent with the history as well as the character of the Federal Constitution… In that mighty struggle between the first intellects of this or any other country it never occurred to any individual, either among its opponents or advocates, to assert or even to intimate that their efforts were all vain labor, because the moment that any State felt herself aggrieved she might secede from the Union.”
Here is another:
In order to justify secession as a constitutional remedy, it must be on the principle that the Federal Government is a mere voluntary association of States, to be dissolved at pleasure by any one of the contracting parties. If this be so, the Confederacy is a rope of sand, to be penetrated and dissolved by the first adverse wave of public opinion in any of the States. In this manner our thirty-three States may resolve themselves into as many petty, jarring, and hostile republics, each one retiring from the Union without responsibility whenever any sudden excitement might impel them to such a course. By this process a Union might be entirely broken into fragments in a few weeks which cost our forefathers many years of toil, privation, and blood to establish.
Here's what he said on January 8th 1860:
In my annual message I expressed the conviction, which I have long deliberately held, and which recent reflection has only tended to deepen and confirm, that no State has a right by its own act to secede from the Union or throw off its federal obligations at pleasure. I also declared my opinion to be that even if that right existed and should be exercised by any State of the Confederacy the executive department of this Government had no authority under the Constitution to recognize its validity by acknowledging the independence of such State. This left me no alternative, as the chief executive officer under the Constitution of the United States, but to collect the public revenues and to protect the public property so far as this might be practicable under existing laws. This is still my purpose. My province is to execute and not to make the laws. It belongs to Congress exclusively to repeal, to modify, or to enlarge their provisions to meet exigencies as they may occur. I possess no dispensing power.
Buchanan was quite clear about what he believed, though people might argue forever about the real meaning of what he said. James Buchanan had passed the bar, but he was not a Constitutional lawyer. His arguments, so far as I have been able to ascertain, don't cite sections of the Constitution, but operate in more general terms. Because Buchanan wasn't in court making a case before judges, his arguments focused more on whether there were valid grounds for secession, not on specific clauses of the Constitution.
But this strongly implies that unilateral secession at will, for any reason or none was not a right retained by the states (indeed, Buchanan says just that in the sentence in bold above). There would have to be a ground for revolutionary actions that went beyond the simple desire of people in any state to be rid of the union, and Buchanan did not believe that such reasons existed. Therefore, secession, not being a constitutional provision, was also not justified as a legitimate act of revolution against unbearable grievances. In any case, it's clear that Buchanan thought that secession would make the Constitution meaningless and that unilateral secession was not a right that states had.
I get the feeling this is a pointless quarrel. When somebody says, "You have absolutely no right to do that, but neither I nor anybody else has the right to stop you," people can argue forever, going around and around in circles, about what the meaning or intention behind those words is. If nobody can stop you, can't it be said that you actually do have a right? And what are we to make of the attitude of the person who finds something wrong but will do nothing to stop it or hinder it? But historians will tell you that Buchanan did not believe that the Constitution allowed for slavery. Because he didn't. And (though it's another endless argument) the Constitution didn't either.
If you want to go on being a neo-Confederate, you shouldn't be too hard on Buchanan. With all his many faults, he did provide an example of how Northerners could cave in to secession without feeling that they had betrayed their convictions. Otherwise, if one truly believed that secession was unconstitutional one would have to take action against it.