Legally speaking the States cannot.
That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.
Was effectively destroyed by the Civil War. I still find it amusing that Lincoln's greatest speech was about how wonderful it was when one nation broke away from another.