Interesting Take

That I suspect is correct: immigration.

PPS. I don’t believe that immigration will make America more diverse, nor do I think it will make the electorate vote more Democratic. That’s because immigration from Asia and Latin America has made earlier immigrants from southern and eastern Europe seem less different, more “white” than they seemed in 1924, when immigration was restricted because America seemed to be becoming extremely diverse. By the 1950s, that same ethnic mix was viewed as homogeneous. Intermarriage rates are now so high that the day will come when both Asians and middle class Hispanics will be viewed as culturally “white”. Fifty years from now America will still be about 75% “white”, in terms of people’s perceptions. (The black population will stay around 13%) Diversity is a state of mind.

And as immigration brings in people who don’t vote Republican, it makes existing residents more likely to vote Republican. Thus each party will continue to win roughly half of the vote.