Without a unifying national mission, Americans have sorted themselves into clans based on social class and culture. Consequently, the two major political parties are divided not just by policy preferences, but also by identity. Rural whites now overwhelmingly vote Republican while most minorities and urban whites vote Democrat. This crude tribalism leaves little room for compromise. If the partisan divide were merely about policy, the parties could split the difference. Now that American politics has become a clash of cultures, however, both parties view compromise with the other side as immoral and dangerous.
Michael Beckley, Unrivaled: Why America Will Remain the World's Sole Superpower