I could maybe see it if the POD were in the mid to early 19th century and the Civil War, or at least West vs East Civil War, didn't happen until the 20th century. Perhaps California and Texas both develop stronger 'national' identities, and then somehow becoming more culturally and economically linked. Maybe there's more immigration, further conquests in Mexico, or earlier western railroad and irrigation development? Maybe you could also see all of the Pacific Northwest develop into a super-state of Oregon along the lines of California and Texas. Then if you had enough population and cultural/economic independence, they lead the rest of the West-of-the-Mississippi in a revolt, maybe over TTL's equivalent of socialism, or because of some corruption seen in the national government.
Not easy, but perhaps imaginable.