Focus on crime to scare voters to the right. Even the most liberal cities in CA will shift Republican if they feel Democrats aren't being tough on criminals (see District Attorney recalls in SF, Alameda County, LA County, etc).
More generally call out the Dem failures of the major cities and actually offer solutions. Intense NIMBYism supported by Democratic politicians in cities like San Fransisco has contributed greatly to the current housing affordability crisis - if Republicans promised to brute force build more housing, I could see them gaining a lot of ground, especially amongst younger folks who don't own homes and feel like they may never be able to afford one.
