Don't forget the 10th Amendment argument. The Constitution goes into considerable detail over how a potential state can join the Union and become a State and what all is required. Nothing is said about how to leave the Union and what is required. No power to stop States from leaving was delegated to the United States. The 10th Amendment leaves undelegated rights to the States.
"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."