Here is another interpretation by James Madison.
LETTER FROM MR. MADISON ON THE RIGHT OF SECESSION.
Madison argues that the ratification of the Constitution was a compact between the states, and as such, each state has equal say in all matters. When one state declares itself to secede unilaterally, it says that its own decision is elevated above all the rest.Madison then suggests that if a state declares its own desire supreme over the others, then that right extends to all the other states too. That means that if a state has a right to secede from the others, then the others have the right to secede from it. In other words, a body of states has the right to oust a state against its wishes, which is a dangerous precedent.
James Madison to Daniel Webster
The argument here is that when the states ratified the Constitution, that conferred United States citizenship on the citizens of the several states (I thought the Declaration of Independence did that) with all the rights and powers laid out in the Constitution. Once given, a governor or legislature does not have the right to strip its state citizens of their citizenship in the United States.
-PJ
It's an interpretation, not an explicit rule in the Constitution.
The precedential holding in the Supreme Court case of Texas v. White, 74 U.S. 700 (1869) is the law of the land until the Constitution is amended, or the Court issues a superseding opinion. That is not weak; that is the law of the land. Rightly or wrongly, it holds that secession is unconstitutional.