And that, in turn, is reflected in the national fiscal outlook. There are now 2.8 workers for every recipient of Social Security benefits, a rate on track to fall to 2.2 by 2035, according to the program’s trustees. Many state pension plans face even greater demography-induced strains.

In smaller cities and rural areas, demographic decline is a fundamental fact of life. A recent study by the Economic Innovation Group found that 80 percent of American counties, with a combined population of 149 million, saw a decline in their number of prime working-age adults from 2007 to 2017.

Population growth in the United States has now hit its lowest level since 1937, partly because of a record-low fertility rate — the number of children born per woman. The direction of United States population growth is increasingly similar to that of slow-growing Japan and Western Europe, with immigration partly offsetting that shift.

The Trump administration has portrayed the surge of asylum seekers at the southern border as a crisis, and applied aggressive tactics to deport undocumented immigrants already in the United States. But it has also announced plans to issue up to 30,000 additional H-2B visas for temporary workers.

“That immigrants keep showing up here is a testament to our freedom and the economic opportunity here,” said Matthew Kahn, an economist at the University of Southern California. If immigrants weren’t trying to come — if they believed the United States to be full — that would be a problem, Mr. Kahn said.

A particular fear, said John Lettieri, president of the Economic Innovation Group, is that declining population, falling home prices and weak public finances will create a vicious cycle that the places losing population could find hard to escape.

He proposes a program of “heartland visas,” in which skilled immigrants could obtain work visas to the United States on the condition they live in one of the counties facing demographic decline — with troubled counties themselves deciding whether to participate.