Democrats Are Wrong About Republicans. Republicans Are Wrong About Democrats.

Welcome to Secret Identity, our regular column on identity and its role in politics and policy.

The defining divide in American politics is probably between Republicans and Democrats. It encapsulates all our other divides — by race, education, religion and more — and it’s growing.

This partisan divide is such a big part of people’s political identities, in fact, that it’s reinforced simply by “negative partisanship,” or loyalty to a party because you don’t like the other party. A Pew Research Center poll from last year found that about 40...