Both 1976 nominees came from their parties’ historic heartlands, the Upper Midwest and the Deep South. That helped Carter reassemble a Democratic coalition that looked much like Kennedy’s, uniting Northern Catholics and Deep South whites, augmented with blacks. Gerald Ford, like Richard Nixon, carried affluent suburbs and California, even winning 49 percent in the San Francisco Bay Area.

But that 1960-76 alignment quickly disappeared. Carter lost 12 of 14 Southern states to Ronald Reagan, and the South became bedrock Republican presidentially in 1984, and congressionally in 1994. Affluent suburbs in million-plus Northern metropolitan areas moved toward Democrats in the 1990s. Mitt Romney got 24 percent in the Bay Area in 2012.

For 2016, Hillary Clinton is trying to reassemble the Obama majority. Republican strategists are hoping to pluck enough 2012 target states to win. But events so far— Clinton’s slide, the Trump phenomenon—may destabilize what have been enduring partisan preferences. And who knows what turbulence lies ahead?