Broadly speaking, there are three trends that we can point to. The first is the steady nationalization of American politics. The second is the sorting of Democrats and Republicans along urban/rural and culturally liberal/culturally conservative lines, and the third is the increasingly narrow margins in national elections.
The combination of these three trends has turned Washington, D.C., into a high-stakes battle where cross-party compromise is difficult, and both sides are increasingly holding out for complete control…
With all these identities accumulating on top of each other, partisanship has become a kind of “mega-identity,” as political scientist Lilliana Mason argues, with party identification standing for much, much more. In fact, it’s reached the point that when you meet somebody, you can immediately size them up as a “Trump voter” or a “Biden voter.” That kind of easy stereotyping leads us to see the other party as distant and different. And typically, things that are distant and different are also more threatening.
But neither side has come to dominate. Instead, America has experienced an extended period of national parity between the two parties. Elections have swung back and forth in an almost predictable pendulum fashion since 1992 — unified control of one party, divided government, unified control of the other party, and so forth, over and over.