In 1855, the California State Assembly passed a plan to trisect the state. All of the southern counties as far north as Monterey, Merced, and part of Mariposa, then sparsely populated but today containing about two-thirds of California's total population, would become the State of Colorado (the name Colorado was later adopted for another territory established in 1861), and the northern counties of Del Norte, Siskiyou, Modoc, Humboldt, Trinity, Shasta, Lassen, Tehama, Plumas, and portions of Butte, Colusa (which included what is now Glenn County), and Mendocino, a region which today has a population of little more than half a million, would become the State of Shasta. The primary reason was the size of the state's territory. At the time, the representation in Congress was too small for such a large territory, it seemed too extensive for one government, and the state capital was too inaccessible because of the distances to Southern California and various other areas. The bill eventually died in the Senate as it became very low priority compared to other pressing political matters.
I found myself interested in some possible state border differences in the course of writing my setting, and this one stuck out to me as a really interesting one. What if it had split back in 1855? Taking a look at how California actually turned out, offhand it looks like the result would be:
1.) Shasta: Northern section. Comparatively rural and conservative, like some of the mountain states.
2.) California: Middle section. Contains the modern silicon valley. Mini-California, basically. Solid economy based on how much of real California's economic production comes from the Bay Area.
3.) Colorado: Southern section. The Hollywood part. (Los Angeles, Colorado...interesting ring to it) Based on actual economic data, this would be the most productive one.
This seems too simplistic to leave things at, though. There is, of course, the issue of The Civil War. Since this trisect happens in the lead up to The Civil War, and Californians originally voted adamantly to be a Free State, it seems obvious a trisect would result in three Free States. From what I've read, the various compromises on the issue were really starting to fray by this point, so maybe this accelerates the issue if it actually went through.
Given how huge California is, one would think a trisected one to have other consequences. California has such a huge economy that many of its rules wind up affecting things outside the state, as it's just economical to do go along with them, for one rather than not be compliant with such a big consumer base. However, given how concentrated that economic power and associated clout is in the southern and middle trisected parts (especially the southern one), I wonder just how much would actually change. There's also bound to be some legislative stuff, but there's so much there I'm not sure where to start.