Eh, that's not entirely true. Trump came to OC and San Diego because there are votes to be gained here. Republicans do pretty well in SoCal relative to NorCal, and the entire eastern 3/4s of the state not including Sacramento is red. I think once you get into the NorCal echo chamber, it's not going to matter much how much time Republicans spend up there. Demographics and hippies don't favor them up there.,
Issue is out of the top eleven populated states four of them (CA/IL/NJ/NY) are states that the Republicans do not campaign much or even bother to have much a of a ground game. Start campaigning, build a ground game, and that one million votes disappears pretty quick IMO.
Voter turnout in a lot of California is already good because we vote on so many ballot measures (20 this year for me, and more like 30 for those in San Diego city) in addition to our candidates.