“In theory, because it was a majority vote of Congress to admit California, in theory a majority vote of Congress could approve secession.” …Erwin Chemerinsky, dean of the University of California, Berkeley, School of Law 2017
http://www.gq.com/story/california-secession-movement
“A state couldn’t legally force the U.S. to let it secede, if the U.S. refused. But if the state and the U.S. agreed, that would be doable; at worst, it would require a constitutional amendment (always possible, though it requires two-thirds of the vote of each house of Congress and a majority vote in three-quarter of the state legislatures), but most likely an act of Congress, coupled with a majority referendum that the state government views as binding, would suffice,” … Eugene Volokh, a law professor at UCLA
https://www.dallasobserver.com/news/heres-why-texit-can-but-wont-happen-8432002
“I suppose theoretically it’s an open question as to whether a state could secede with the consent of Congress or with the consent of all the other states. There’s some throwaway language about the possibility of all states agreeing to allow one state to secede in Lincoln’s First Inaugural Address and in Texas v. White,”…Cynthia Nicoletti, professor of law at the University Virginia School of Law
https://www.newsweek.com/american-states-that-wish-join-russia-will-considered-says-duma-member-1750336
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