Article 4, Section 3 of the Constitution states:New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.This is really not feasible without direct assent from the state being separated from.  Both Maine and Kentucky split based on the assent of Massachusetts and Virginia under the Constitution.West Virginia was a wartime ploy, an act that held up due to the acceptance as fact that enough representatives of the state of Virginia showed up at the Wheeling Conventions to justify a state legislature.  Lincoln and Congress accepted it, and in 1863, West Virginia was accepted as a state separate from Virginia.  In peacetime, this arrangement would never have worked.To get the obstinate California legislature to go along with the notion of several of their prosperous counties just leave is just impossible.