Republican candidates for the presidential nomination claim that Democrats kill babies and harvest their organs to sell them, insist the U.S. is at war with an "evil state of consciousness," compare Muslims to rabid dogs, and call for closing mosques and registering Muslims. These are not fringe candidates. They are the front-runners. American politics has descended from principle into tribalism. The descent began in 1968. That year's presidential election looked to be a principled fight, with Democrats Hubert Humphrey and Robert F. Kennedy articulating a vision of an inclusive America in which the government expanded efforts to guarantee equality. For...