American politics has been dominated by the Democratic and Republican Parties since the Civil War. That gives us the illusion of stability — that today’s political divisions cut roughly the same lines as yesteryear.
But in recent decades, the two parties have been changing, and fast. Those changes are ideological — the Democratic Party has moved left, and the Republican Party has moved right. But more fundamentally, those changes are compositional: Democrats have become more diverse, urban, young and secular, and the Republican Party has turned itself into a vehicle for whiter, older, more Christian and more rural voters.
This is the root cause of intensifying polarization: Our differences, both ideological and demographic, map onto our party divisions today in ways they didn’t in the past. But the changes have not affected the parties symmetrically.The New York Times