President Trump’s pledge to revive American manufacturing is running into the stubborn obstacle of demographic reality.

The pool of blue-collar workers who are able and willing to perform tasks on a factory floor in the United States is shrinking. As baby boomers retire, few young people are lining up to take their place. About 400,000 manufacturing jobs are currently unfilled, according to the Bureau of Labor Statistics — a shortfall that will surely grow if companies are forced to rely less on manufacturing overseas and build more factories in the United States, experts say.

Difficulty attracting and retaining a quality work force has been consistently cited as a “top primary challenge” by American manufacturers since 2017, said Victoria Bloom, the chief economist at the National Association of Manufacturers, which produces a quarterly survey. Only recently has the issue slipped down on the list of challenges, superseded by trade-related uncertainty due to the Trump administration’s tariffs and by increased raw material costs, Ms. Bloom said.

But the scarcity of skilled blue-collar workers remains a long-term problem, according to Ron Hetrick, an economist with Lightcast, a company that provides labor data to universities and industry.

“We spent three generations telling everybody that if they didn’t go to college, they are a loser,” he said. “Now we are paying for it. We still need people to use their hands.”

The hiring challenges faced by American factories are multifaceted.

The president’s crackdown on immigration, which includes attempts to revoke deportation protections for migrants from troubled countries, may eliminate workers who could have filled those jobs.