The Biden administration on Friday issued sweeping expansions of deportation protections for hundreds of thousands of people from Sudan, Ukraine, El Salvador and Venezuela. This makes it nearly impossible for President-elect Donald J. Trump to quickly revoke deportation protections if he uses his office.

The extension of Temporary Protected Status, as the program is called, allows immigrants to remain in the country with work authorization and protection from deportation for an additional 18 months after their current protection expires in the spring. Late last year, Secretary of State Antony J. Blinken recommended expanding protections in a series of letters.

For decades, Democratic and Republican administrations have established protections for citizens of countries in transition and whose return is considered unsafe. President Biden expanded who could receive the status as war broke out in Ukraine and instability gripped countries like Venezuela and Haiti.

“These designations are based on careful consideration and cross-agency collaboration to ensure those affected by environmental disasters and instability receive the protections they need while continuing to make meaningful contributions to our communities,” said Rep. Adriano Espaillat of New York , chairman of the Congressional Hispanic Caucus.

Mr. Trump has vowed to end the program, at least for certain countries. Immigration advocates had called on the Biden administration to expand it to many of these countries before he takes office.

In his first term, Mr. Trump ended the status of about 400,000 people from El Salvador and other countries, saying conditions there had changed and protections were no longer warranted. The move was challenged in court and did not take effect, but he is expected to try again in his second term as part of his promise to carry out mass deportations.

According to the Congressional Research Service, more than one million migrants from countries in Latin America, the Caribbean, Africa and the Middle East had temporary protected status in 2024.

The move makes it legally difficult for Mr. Trump to roll back protections for citizens of the four countries, at least until they expire sometime in 2026.

“Because President Biden has expanded protections for nationals of all of these countries, President Trump will not be able to deport these individuals any time soon,” said Steve Yale-Loehr, an immigration scholar at Cornell Law School.

“Trump cannot ignore what Congress signed into law in 1990,” he said.

Approximately 600,000 Venezuelans who currently have the protection will be allowed to extend their protection and remain in the United States until October 2026, and approximately 232,000 immigrants from El Salvador will be able to do so. More than 100,000 Ukrainians will also be able to stay in the US until October 2026. Around 1,900 people from Sudan are also allowed to extend their status.

The program was enacted by President George H. W. Bush to ensure that foreign citizens already in the United States can remain in the country if they are unable to return to their home country due to a natural disaster, armed conflict or other civil unrest she is not sure.

During the campaign, JD Vance, the vice president-elect, called the program illegal as he criticized Haitians who had settled in his home state of Ohio and benefited from it. Haiti has experienced political unrest and gang violence, and around 200,000 of its citizens are protected from deportation through TPS until early 2026.

“We will stop granting temporary protected status en masse,” Mr. Vance said in October.

Critics have argued that the temporary protection measures are repeatedly extended and serve as a de facto means of allowing people to stay in the country indefinitely, contradicting the intent of being a short-term solution.

While the program has become all but permanent for many immigrants, it also highlights how problematic many parts of the world are and how Congress is failing to pass legislation to adapt the U.S. immigration system to the realities of today’s global migration.

Immigrants from several countries, including El Salvador, Honduras and Nicaragua, have been eligible for the protections for more than two decades. Other countries such as Ethiopia, Lebanon and Syria have recently been added.

If this status were eliminated, hundreds of thousands of immigrants would immediately become illegal residents of the United States unless they left immediately. Many of them have U.S.-born children, businesses and jobs in sectors that rely on immigrant workers, such as construction, hospitality and health care.

In cities like Denver, temporary status has allowed thousands of Venezuelans who arrived from the southern border on buses driven by Republican Texas Gov. Greg Abbott over the past two years to work there legally and integrate into the economy.

Mike Johnston, the city’s mayor, said he welcomed the Biden administration’s announcement to extend the designation.

“In Denver, people with temporary protected status hold essential jobs, contribute to our economy and become integral members of our communities,” he said.

Gonzalo Roa, 43, a Venezuelan who is a beneficiary in Columbus, Ohio, said he was worried about the fate of the program.

“It’s great news that it’s being renewed,” said Mr. Roa, who works at a car dealer and runs a small restaurant with his wife.

Without the status, Mr. Roa said, he would lose his job at the car dealer and his two Venezuelan-born children would be ineligible for college scholarships and other benefits that require legal status.