The gradual sorting of partisans into the “correct” parties during the last fifty years has transformed a nation of cross-cutting political identities into a nation of increasingly aligned political identities. As Democrats and Republicans grow socially sorted, they have to contend not only with the natural bias that comes from being a partisan but also with their own growing intolerance, sharpened by the shrinking of their social world. A conservative Democrat will feel closer to Republicans than a liberal Democrat would. A secular Republican will feel closer to Democrats than an evangelical Republican would. The sorting of our parties into socially distinct groups intensifies the partisan bias we’ve always had. This is the American identity crisis. Not that we have partisan identities, we’ve always had those. The crisis emerges when partisan identities fall into alignment with other social identities, stoking our intolerance of each other to levels that are unsupported by our degrees of political disagreement.
Mason, Lilliana. Uncivil Agreement: How Politics Became Our Identity. University of Chicago Press, 2018. p. 62-3