Matthew Yglesias predicts that the white voters will become more Republican as nonwhites become a greater share of the population:
I used to hold to the view that the growing non-white share of the electorate would, over time, tip elections to Democrats. I now think the system will remain near equilibrium and what we’ll instead see is white voters growing more Republican as Democrats are more and more seen as the party of non-whites. Mississippi and Arizona, after all, have very large minority votes but they’re hardly hotbeds of liberalism. Instead they’re hotbeds of very conservative white people.
Jonathan Chait isn't so sure and -- noting the significant liberal advantage among younger whites -- predicts a relatively durable Democratic majority going forward:
For Yglesias's thesis to be born out, those white voters would have to undergo a massive pro-Republican shift as they age -- not only getting more Republican, but getting more Republican than current old white voters are right now.
That seems unlikely. It seems to me that the choices really are that Republicans will achieve parity among Latinos -- which probably depends upon getting them to identify with the whites and against the blacks -- or else Democrats will have a general working majority, interrupted by recessions, scandals, and whatnot.
While Chait is probably underestimating the extent to which a large number of whites will become more conservative as the nonwhite population grows (most white people aren't under the age of 30), in the long-term, he's probably right, especially given the tendency of partisan affiliation to harden after a certain point. In all likelihood, if you're a Democrat in your20s, then -- barring a dramatic change in the political landscape -- you'll probably be a Democrat for the rest of your life.
Two additional things.
First, I'm still pretty bullish on the possibility that Republicans will achieve parity with Latinos; as I've noted before, given high intermarriage rates between whites and Latinos, there's a fair chance that future America will have a large population of white people with Hispanic last names. Given the fluidity of these things, there's no real reason to think that these whites wouldn't be susceptible to conservative appeals on national identity/American-ness. Indeed, the only complicating factor is the Republican Party; continued demonization of Latinos as an electoral strategy could easily lead to a situation where most Latinos move overwhelmingly to support most Democrats in most elections (see: African Americans for a current example of this).
Second, even if Republicans do gain a foothold among Latinos, my guess is that the Democratic Party will still end up with a general working majority in most instances. It's not that Americans love liberalism, but that Democrats are willing to tolerate huge levels of ideological difference for the sake of electoral gain. Between demographic changes and partisan coalitions, I wouldn't be shocked if -- over the next 50 years -- we return to the 20th century pattern of "natural" Democratic majorities, interrupted by brief periods of conservative dominance and the occasional Republican president.