Employers are having trouble attracting qualified applicants to blue-collar jobs, a dynamic that is complicating President Donald Trump’s promises to revive American manufacturing. The Bureau of Labor Statistics estimates that 400,000 factory jobs are currently vacant in the U.S.

As baby boomers retire, some younger workers are unwilling to give up the flexibility, pay and relative comfort of service jobs to work in factories. Trump’s immigration crackdown may further diminish the pool of available blue-collar workers, some company executives say.

“For every 20 job postings that we have, there is one qualified applicant right now,” David Gitlin, chair and CEO of Carrier Global, a heating and air conditioning company, told The New York Times. That gap may widen even more if trade policies push more companies to manufacture goods in the United States.

There is currently a mismatch between available workers and their skills: Many of the college graduates struggling to find work do not have the training to work on factory floors.

And though Trump has signed an executive order directing Cabinet officials to make a plan to create 1 million registered apprenticeships, his administration has also moved to shut down dozens of Job Corps centers, which give at-risk young people a path to working in trades.

Business leaders have focused on efforts to train veterans and introduce high school students to the idea of working in manufacturing.

“We spent three generations telling everybody that if they didn’t go to college, they are a loser,” Ron Hetrick, an economist with Lightcast, which provides labor data to universities and industry, told the Times. “Now we are paying for it. We still need people to use their hands.”