That wouldn't happen. San Francisco would never consent to being separated from its suburbs and thrown in with Los Angeles.
But even if it did, Sacramento and the liberal coastal counties would probably give Northern California to the Democrats. There just aren't that many people in inland Northern California.
In a good year, Republicans might be able to elect a governor or senator in the Southern California state, but the other two would be solid Democrat.
I think it is time for City states to be formed separated from the states around them. They can elect reps and one senator if they are over 75% citizens of the us.
Otherwise they lose the senator and 25% of the reps.