WASHINGTON — At a private event last week, Mick Mulvaney, the acting White House chief of staff, stated a reality that economists treat as conventional wisdom but that the Trump administration routinely ignores: The United States needs immigration to fuel future economic growth.

“We are desperate, desperate for more people,” Mr. Mulvaney told a crowd in England. “We are running out of people to fuel the economic growth.” He said the country needed “more immigrants” but wanted them in a “legal” fashion.

Mr. Mulvaney’s sentiments are at odds with President Trump’s crackdown on undocumented entries and family-based immigration into the United States. But they reflect the viewpoint of economists and many in the business community, who say that immigrants are needed to power the U.S. labor market as growth in the native-born work force rapidly slows as the population ages and people have fewer children.

Immigrants have already been a crucial source of new workers, accounting for about half of the labor force’s expansion over the past two decades. But the foreign-born population has been expanding only tepidly during Mr. Trump’s tenure. That slowdown could have long-lasting and profound repercussions, economists warn.