What the trends on affective partisanship suggest is that even if members of the two parties are not especially far apart on substantive issues, their social, cultural, racial and ethnic conflicts and differences will preclude agreement on the selection of a centrist candidate as the presidential nominee of either party.

The pre- and postelection extremism of Trump and his loyalists have spurred a modest backlash on the right that could, over time, prompt the party to abandon its notions of betrayal and victimization.

But for the moment, the signs of retrenchment are modest. The country’s conservative party is wedded to an extreme position — with an astonishing 59 percent of Republicans convinced as recently as March 5-9 that Joe Biden is not a legitimately elected president, according to a YouGov poll.

When one party sinks that far into delusion, cross-party cooperation is ruled out, and the kinds of centrist policies that many voters say they want become an impossibility.