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 their part, Republican managers understood that they had a problem. The two sides of the Republican Party were too far apart to be cinched together by any national vision.

On the one hand, moderate Eisenhower voters believed in using the federal government to promote equality of opportunity, although they were nervous the Johnson administration’s War on Poverty had gone too far. On the other hand, Movement Conservatives who had backed Barry Goldwater in 1964 rejected the principles of the New Deal. They wanted the government to stop meddling with the social welfare legislation that they insisted was a redistribution of tax dollars from hardworking white people to lazy African-Americans.

To win the 1968 election, Richard Nixon’s team adopted the Southern Strategy, sacrificing black rights to cement Movement Conservative white voters to the Republican Party.

The racial dimension of this decision is well known. Less well known is that it also marked a seismic shift in the mechanics of American politics. The chaos of 1968 cemented the Republicans’ new electoral strategy. Dr. Martin Luther King Jr. and Robert Kennedy were assassinated. Democratic presidential nominee Hubert Humphrey got sent into obscurity, and former Vice President Richard M. Nixon won the White House without a popular majority. ...