President Trump continued his sweeping crackdown on immigration on Friday, turning his focus to a visa program for skilled foreign workers. He signed an executive order that adds a $100,000 fee for new applicants for H-1B visas that allow foreign workers like software engineers a chance to be employed in the United States.

The H-1B visa is designed to help companies fill openings for which American workers with similar abilities cannot be found. But immigration hard-liners and far-right activists have long argued that the visa allows companies to replace American workers with foreign ones. The issue has divided even Mr. Trump’s supporters, and the president’s own stance on the program has shifted over time.

Ahead of the new order’s signing in the Oval Office on Friday, Howard Lutnick, the secretary of commerce, laid out the rationale for the fee that the administration is attaching to what he called the “most abused visa.”

“The whole idea is, no more will these big tech companies or other companies train foreign workers,” Mr. Lutnick said. “They have to pay the government $100,000, then they have to pay the employee — so it’s just not economic. If you’re going to train somebody, you’re going to train one of the recent graduates from one of the great universities across our land.”

The fee is likely to face legal challenges. It is slated to go into effect Sept. 21 and will only be required for new applicants, according to a memo on Saturday from U.S. Citizenship and Immigration Services.

Here’s what you need to know.

What is the H-1B visa program?

Congress passed legislation creating the H-1B program in 1990, as a labor shortage loomed. When George Bush signed it into law, he said the program would “encourage the immigration of exceptionally talented people, such as scientists, engineers and educators.”