What a very bad idea. Furthermore, any areas outside of SF, LA, and a few other coastal towns, the rest of the state would not break off.
I agree.
What we have is a group of 3 to 4 rebellious city-states run by Democratic Party machines. If a the Civil War between America and California occurs the coastal cities will be put under siege and starved out. These cities will become Mad Max, dystopian hellholes that will induce the populace to flee for their lives. The city states will fall with a whimper and most of the damage will be caused by the rebels themselves.
Also, if war between California and the United States occurs, Mexico will not be spared and they will lose even more territory. It wouldn't be unreasonable to imagine that the entire west coast of Mexico could be taken over.