When Donald Trump was reaching adulthood in the mid-1960s, the United States was a less diverse place. By 1970, the share of the population born overseas had shrunk to 4.7 percent, the slimmest on record. Only about 0.4 percent of the population had been born in Mexico.

The core constituency of Mr. Trump, the Republican front-runner — white, older voters like him who are more likely to believe that immigrants take Americans’ jobs, housing and health care than accept that they contribute to the economy — came of age largely at that stage in history, from the 1950s through the early 1970s.

“It was a unique period of rapid economic growth, when the children and grandchildren of Europeans were blending into a homogeneous mass,” said Douglas S. Massey, a Princeton sociologist. “That world is gone.”

This transformation provides the most convincing explanation of the runaway popularity of Mr. Trump’s proposition to kick out the estimated 11 million immigrants living illegally in the United States today and close the door to future migrants with a 2,000-mile border wall.