The California secessionists wouldn’t be successful in trying to take the entire state. Military bases would remain under U.S. control, so Camp Pendleton, Edwards Air Force Base and other military bases would be excluded. After also excluding any sane areas of the state that want to remain in the U.S., the new California would probably be a weirdly-carved area largely along the coast from L.A. (or maybe northern Orange County) up to and slightly above the bay area, running east to Sacramento and then kinda sorta down the I-5 corridor.
New California doesn’t get access to the water from the Sierra Nevadas or the Colorado River.
I would think there are few voters up and down the I-5 from Stockton to Valencia. And the valley is pretty conservative. If the dems are in charge the cattle at Harris Ranch may register to vote for succession. They are the largest demographic in the corridor.