The Constitution is silent on a state’s ability to secede so the Supreme Court essentially legislated from the bench in Texas v. White (1869) declaring secession illegal which avoided a Constitutional basis. So much for the sanctity of the Judicial Branch! So we are left with what Mao said, “ power comes from the end of a gun”.
As the Declaration of Independence clearly states that people have a right to abolish a government that does not suit their interests and form one that does, the burden of proof shifts to those who claim the constitution forbids it.
We don't have to show the constitution allows it, the opposition has to show the constitution prohibits it, and all the evidence I have seen on the topic indicates the framers believed states could leave if they so chose.
There is very little evidence to support the opposite claim. Mostly just two letters from Madison, written 40 years apart, and contradicting his own verbiage in Virginia's ratification statement.