For nearly three decades, Raquel Molina — an immigrant from El Salvador who has a valid Social Security number and permission to work in the United States — swabbed the toilets, wiped down the seats and vacuumed the aisles of airplanes at Boston’s Logan International Airport.

But last summer, Ms. Molina, 65, was abruptly fired from her $19.75-per-hour cleaning job, alongside dozens of other immigrants who have long legally worked at Logan. Her supervisor told her she no longer had clearance to enter secure areas at the airport. The Trump administration had decided that only U.S. citizens, green card holders and others with more permanent forms of residency should be granted access, according to a lawsuit that a labor union filed in federal court.

“I didn’t understand what was going on,” said Ms. Molina, who has been living legally in the United States under Temporary Protected Status, a humanitarian program that shelters people from troubled countries until they can safely return home. “I had worked hard at my job. This news put me in a state of shock.”

Her firing reflected a broader and methodically planned piece of President Trump’s hard-line strategy to make the United States less welcoming to those from other countries.

For more than a year, administration officials have sought to pull every bureaucratic lever possible to cut off immigrants — both documented and undocumented — from jobs, medical care, financial services, tax credits and even from enrolling their children in day care. The goal has been to compel immigrants to leave the country, and, in the long run, to eliminate incentives that draw many people to the United States in the first place.

The initiative underscores the president’s ability to reshape immigration policy through executive orders and the vast power of federal regulations while sidestepping Congress. And it shows how the administration has pursued more creative — and lower-profile — tactics after Mr. Trump’s militarized deportation raids into major cities prompted political backlash earlier this year.