Somehow stop the Hispanic "invasion" from Mexico and Latin America, or at least NEVER let the Hispanics become voters. I have no idea how you'd accomplish the former, but the latter is legislative.
Back in the day, California voting Republican used to balance New York voting Democrat. Then the Hispanics came and vote ~ 100% Democrat.
That is true to a point, but other demographic shifts occurred. While Hispanics arrived in great numbers attracted by the same opportunity as tens of thousands of emigrants from other states, California was subject to large amounts of White Flight. High property values, and the development of the Southwest/Mountain-West attracted thousands of mostly white and conservative Californians.