The U.S. must retrain older and lower-skilled people in order to maintain the current pace of job growth, White House economists argued Tuesday in a new paper that makes the case for apprenticeships and greater spending on retraining programs.

“There simply are not enough unemployed workers in the current pool of those looking for work to match the growth in demand for new workers unleashed by the Trump administration’s pro-growth policies,” President Trump’s Council of Economic Advisers wrote in a 29-page analysis. “Rather, employment gains must also come from working-age Americans returning to employment from the sidelines.”

Trump’s economic advisers have consistently pushed for bringing more people into the labor force. Last week, they published a paper finding that a majority of beneficiaries of safety-net programs are candidates for work requirements.

The question of labor force participation is a timely one, as the unemployment rate has fallen to the lowest it has been in decades. In the past few months, there have been more advertised job vacancies than unemployed workers, a situation that hasn’t been seen in decades.

The new paper makes the case for the government and businesses to spend more on training workers, noting that employers and governments spend much more educating and training young people than older people who might be candidates to take new jobs. It also notes that the U.S. spends far less than peer countries on training programs.

The paper floats several ideas for financing new retraining ventures. One would be to allow unemployment insurance benefits to be dedicated toward paying the cost of apprenticeships. The Trump Labor Department has sought to boost apprenticeships.

Another option would be to allow government grants, along the lines of Pell Grants, to be used for older workers seeking retraining, rather than just younger students pursuing education.

[Also read: June beats expectations with 213,000 new payroll jobs, unemployment at 4 percent]