Southern California is most similar to Arkansas or Oklahoma demographically/historically
This is clever but I don't think it's accurate anywhere except Bakersfield. When southern California was peopled, it was by Midwesterners coming from the belt between Ohio and Iowa. You can see this influence in the very Midwestern way that so many of the communities east and south of Los Angeles have their own Protestant colleges, mostly established in the late 1880s: La Verne, Whittier, Pomona, Redlands. Northern California was not settled by these people and does not have many schools like these.
Oklahomans who came to southern California in the 1930s were viewed by the residents already there as little better than Mexicans. Of course they came in large numbers, but that burst of immigration did not last very long and outside of Bakersfield they did not define the local culture, because that was there before they were. From a political standpoint, the region's long Republican history has a lot more to do with Iowa than it does with Oklahoma.
