Hm?....Then secession today would likely be impossible.
Difficult, as it should be, but not impossible.
Remember, our Founders considered there to be two legitimate reasons for "disunion".
The first was "mutual consent", meaning with the approval of Congress.
The second was, in effect, "breach of contract" such as "oppression", "usurpations" and "injury".
Such breaches made "mutual consent" irrelevant and unnecessary.
The issue then, much debated, was who could legitimately declare when such a breach had happened?
But in November 1860, when the slave-power began organizing to declare secession, all that was mute -- there was neither "mutual consent" nor "breach of contract".
Instead there was one event only: the 100% legitimate election victory of Abraham Lincoln and his "Black Republicans" -- a victory which had been entirely engineered by slave-power's own Fire Eaters -- see previous posts on this.
So, what about secession today?
It would certainly require a majority in Congress and/or a Supreme Court ruling of, in effect, constitutional "breach of contract".
But such votes could only be made in support of a political majority, a majority which could then readily correct whatever grievances it felt against current conditions.
And so, ironically but imho intentionally: secession is only politically possible at the moment when it becomes unnecessary to solve political problems.