President-elect Donald J. Trump has vowed a crackdown on immigration like never before.

While his hard-line rhetoric about illegal immigration harks back to his first campaign, one of the president-elect’s targets this time is a decades-old program providing temporary legal status to about one million immigrants from dangerous and deeply troubled countries such as Haiti and Venezuela.

Known as Temporary Protected Status, the program was signed into law by President George H.W. Bush to help people already in the United States who cannot return safely and immediately to their country because of a natural disaster or an armed conflict.

But for some immigrants, the program, which allows them to work legally, has become all but permanent, a reflection of how troubled many corners of the world are and how little Congress has done to adapt the U.S. immigration system to the realities of global migration in the 21st century.

About 200,000 people with T.P.S. are from Haiti, a long-troubled island nation where the assassination of the president in 2021 led to the collapse of the government and the killings of thousands of people by gangs that now control much of the country. Haitians have emerged as the focus of Mr. Trump’s threats to effectively end the program after he and his running mate, Senator JD Vance, spread false rumors that Haitians who have settled in Springfield, Ohio, were abducting and eating pets.

Thousands of Haitians have settled in the city, and the majority of them have lawful status, often through the program. That has made them attractive to local industries in need of workers. But the influx has strained resources and caused friction among some residents, and Mr. Trump seized on those tensions, vilifying the Haitians who have made Springfield home and threatening to effectively end the program for them and hundreds of thousands of other immigrants.