It would be surprising if Texas does not stay Republican for a long time.
On the other side California went from supporting their local son Reagan in 1984 by a margin of 16.24% to supporting Clinton just 8 years later, in 1992, by a margin of 13.4%. Meaning California shifted its partisan affiliation by a margin of almost 30% in just 8 years' time. Now I'm not insinuating that the same will happen in Texas of course - absolutely not - but it just goes to show how unpredictable politics can be and how fast circumstances can turn around and change. I personally wouldn't be shocked if Hillary were able to win the state in 2020, although it's more likely that she won't.
California presidential election results: http://en.wikipedia.org/wiki/Politics_of_California#Political_parties
But the country also swung about 24%. California certainly has trended left, but it was already to the left of the national popular vote in 1984. (+16% for Reagan in CA vs. +18% nationally).
In the near future, I only see Texas voting for the Democrat in a massive Democratic blowout nationally. Texas whites are overwhelmingly conservative, and don't seem to be moving much. Texas Hispanics are more conservative than Hispanics elsewhere. Also, I don't see a large influx of liberals moving into the state that would outweigh the influx of more conservatives.
It's true that before the 1980s, Texas usually voted more Democratic than California. However, the California electorate is more diverse, and has had the type of swing voters that would have trended left recently. In Texas though, whites are mostly the inelastic conservative type that went from loyally voting Democrat to loyally voting GOP now. Maybe the growing minority population could help, but I doubt it would make much of a difference anytime soon.
Arizona has more potential to flip. Although it has a smaller Hispanic population, Arizona whites are less religious and more independent-minded than Texas whites, and Arizona is not as rigidly partisan.
