I'd get pretty aggressive with the gerrymandering and attempt to squeeze all the majority RAT voting bastions in the state into a single ultra-RAT state that would be based along the coastline. Looking at the electoral maps from 2000-2016, the commie "red" counties on Impy's map for the People's Socialist Republik of Kalifornia is a good starting point. Ideally I'd also have it snake into some nearby "swing" counties (Fresno County, etc.)and add certain RAT enslaves into the People's Socialist Republik of Kalifornia.
This new marxist state along the coastline would ideally be reduced down to territorial status so they couldn't have electoral votes or voting representation in Congress. Or better, secede from the United States all together since I'm sure they don't want to live in a country ruled by Donald Trump and a GOP Congress. It would be awkward for Hollywood, L.A., etc. to be in a "foreign" nation, but meh. San Fransicko is the ideal capital city for those freaks.
What's left in inland California would be divided into two states, a northern one and a southern one. One would be very GOP friendly turf and the other would be solid Republican.
Northern California without any of the coastal counties (except Del Norte) and neighboring Obama-majority counties would be fairly easy to carve out. As Impy noted, a few areas like Alpine have lost their minds lately and gone hard for the RATS, but they have such small populations they're not likely to exert any influence over the whole state. Jefferson is a good name for the state, though I find Fieldmarshaldj's suggestion of Yreka for the new state capitol to be bizarre. It has such a tiny population that the capitol of dinky little commie state Vermont has the nation's smallest capitol city but its STILL a bigger state capitol (Montpelier, 7,855 vs. Yreka's 7,290), and Yreka is a totally remote wilderness town that would be unsuited as the capitol city of a major state, both in geography and population. The sparse nature of northern California without the coastal areas make it difficult to find a suitable capitol (its like Nevada without without the Las Vegas region). Still, some place like Yuba City (population: 66,000) would make a lot more sense and have the infrastructure and resources needed to house the state government. Redding (pop. 90,000) might work too.
Southern California minus the marxist coastal areas is much more trickier. It's huge in population and even adding counties that often vote GOP will result in a huge influx of RAT voters into the new state and make it winnable for the RATs. Ideally, you would really have to start carving up individual countries and adding specific strong GOP areas to the state and putting RAT friendly areas into People's Socialist Republik of Kalifornia, and you couldn't do that without a bizarre looking new state that's shaped into all sorts of directions like West Virginia and Maryland, and has non-congruent pieces like Michigan's upper peninsula and that little chunk of Virginia that's separated from the rest of the state. For example, you'd have to carve out the iconic GOP areas of Ventura county like Thousand Oaks (Tom McClintock's hometown) and Simi Valley (home of the Reagan Presidential Library specifically selected because its staunchly conservative) and separate them from the rest of Ventura County. You'd also have to somehow connect Imperial County to the People's Socialist Republik of Kalifornia, even though Imperial County is on the eastern edge of California and the People's Socialist Republik of Kalifornia is along the west coast. It seems to be fairly easy to pick a city to serve as state capitol of this new state (Sierra or Reagania or whatever) Bakersfield has over 300,000 people and is in the ideal place geographically, as well as being rated the most conservative city in California and the 8th most conservative in the United States overall. Very few existing conservative states even have conservative state capitol cities, but Sierra/Reagania could easily have one. Again, Fieldmarshaldj's suggestion of dinky little Barstow in the middle of nowhwere makes no sense logically.
To address Yreka and Barstow as my capital picks, one: Yreka was proposed as the capital for Jefferson a long time ago, so I didn’t pull it out of my butt.
Barstow is at a key point on the interstates. If you’re coming from the east or northeast, you’ve got to go through there to get to the coast (unless you’re coming from the south to San Diego). It would make sense to place it there.
As for requiring a capital to be of a certain larger size, why ? I think putting them in a small town is better. People there should go for work and then leave to go home. Washington, DC was a literal swamp in early years. Nobody wanted to stay there for long and they were anxious to go home. As soon as it was civilized and socialized, it became an attractive place to stay. Now people go there and don’t ever want to leave, save for the few people still possessing their ethics and morals. How many people go to Montpelier and want to stay there for perpetuity ?
As soon as DC started exploding in growth in the mid-19th century (or post-Civil War), the U.S. Capital should’ve been relocated to rural central Kansas or Nebraska. A place that could only accommodate a modest population. Certainly not in Kansas City, Omaha or even Denver just because it was the largest available place in the region. That’s what you want to avoid. Founding Fathers (not just of America, but of individual states) knew that capitals were generally supposed to be secondary in size and in the middle of a state, otherwise they’d have kept them in New York City or Detroit (the original capital of Michigan).
I’ve been to Barstow, spent the night there (the day after fleeing the L.A. riots of ‘92). It’s dusty, windy and has tons of trucks and motels. It’s a perfect place for a capital to be to keep the people there but for a short time and to head home. Yreka is marginally better, and has interstate access, both important.