Four times in our history counties seceded from an existing state to form a new state... most notably the western counties of Virginia, loyal to the Union seceding from Virginia and becoming West Virginia in 1863.
In Reynolds v. Sims (1964): The dictatorial Earl Warren court destroyed the republic model by declaring the U.S. Constitution "unconstitutional" for the states, the single greatest grab of federal power since the 16th and 17th Amendments! This “fatwa” was not just an unconstitutional usurp of state rights, but was anti-constitutional... the so-called "one man one vote" model (direct contradiction of Article IV, Section 4 guarantee of a "republican form of government" for states). The Court hijacked the republic and the cowardly politicians allowed them do it. It may take a constitutional convention to remedy this coup... but president Trump, an equal defender of the Constitution could by executive orders, begin to set-aside these court cases and others that are also anti-constitutional (like Roe v. Wade and Obergefell v. Hodges “gay” marriage ruling). Such actions by President Trump would create a nuclear fire storm of course, but it would call out the politicians to do their constitutional duty. And since the media/liberals/RINOs melt-down at anything Trump does, what would he have to lose?
The #RustyIrony of "Calexit" is the Lefties desire to secede from the U.S.A. could "start the engine" of "red" counties then seceding from California... and maybe those of Oregon and maybe even Washington following. There are enough "red" counties in each of those states to form a contiguous state border for the state of Jefferson.
In California, Oregon, Washington and most "blue" states a single urban area had a larger margin of victory for Hillary in 2016, and Obama in 2008 and 2012, than they won the state by at large. This is truly governing and "taxation without representation".