Almost near-ASB unlikely, something that's been settled law since the end of the Civil War. (
see Texas v. White, 74 U.S. 700; 19 L. Ed. 227, 1869) According to the holding in that case, the Union is perpetual, and secession from the Union is illegal unless a state is permitted to withdraw from the Union with the consent of the other states- there's no defined procedure for doing so, presumably it'd involve some sort of vote passing both houses of Congress, possibly paralleling the procedure for admitting a state, although it's uncertain whether that vote would be by a majority of states or individual members of the houses, and it's unlikely that the rest of the states would agree to chopping bits & pieces off the country, especially ones as economically important as CA, no matter what they might think of them unless something seriously, seriously went off the rails (i.e. the country's already disintegrating or a civil war's kicking off by that point)
An attempt to secede without such permission is playing with fire and has a chance of starting a civil war, as a state doing that and its officials would be acting illegally, and any attempt to forcibly resist the Federal government's efforts to assert its lawful authority would be armed rebellion, essentially an effort to gain a new status by waging war against the United States, and therefore an act of treason. (And yes, I believe that Jefferson Davis, Robert E. Lee, et. al., down to the lowliest file clerks and privates in the Confederate government & military were traitors who got away with it because of the practical political necessities of trying to reintegrate the southern states back into the Union without provoking a prolonged insurgency.)