With respect to the possibility of secession by mutual agreement, we are left in much the same position that Americans in the first seven decades of the Union occupied with respect to unilateral secession: We must struggle to interpret the sounds of the Constitution’s silence.
That conclusion in turn suggests that no court will likely answer the question—except perhaps in the way that the Supreme Court in Texas v. White gave its retroactive approval to the verdict of the Civil War battlefield.
This is where the movement gets into crazy town.
Despite their rhetoric about the permanence and indestructibility of the Union, both Lincoln in his First Inaugural, and the Supreme Court in Texas v. White, strongly implied that it would be possible for one or more states to leave the Union with the consent of the Union as a whole.
By what legal mechanism would such secession through mutual agreement be accomplished? The most obvious answer is a statute enacted by Congress. Just as Congress can approve the admission of new states, the argument would go, so it can let old states leave.
If Texas secedes, will the Feds come and try to force them back into the United States?
It's called "war", and it's been implemented in the past.
Amendment X - Powers not specifically delegated to the Federal Government are reserved to the States and People (paraphrase)
If there is not an article delegating the power to the Federal Government to prevent a State from leaving, then that power doesn’t exist.
Too bad Lincoln couldn’t comprehend that.
Doesn't the 10th Amendment mention how to interpret the sounds of the Constitution's silence?