By noon, hundreds of lawyers were interviewing relatives and friends of travelers who were being held [because of Trump’s travel ban], challenging their detention and drafting petitions for their release.

The mobilization that morning in 2017 spawned a network of hundreds of lawyers who are now ready to fight the crackdown on immigrants that Mr. Trump promised to carry out in a second term in office.

After his decisive victory over Kamala Harris, Mr. Trump is expected to name key cabinet choices in the coming days and weeks, including his nominee to lead the Department of Homeland Security.

The Supreme Court upheld a version of the ban on travelers from several predominantly Muslim countries, which the Biden administration eliminated in 2021. But earlier this fall, Mr. Trump said he would “bring back the travel ban.”

During his campaign, Mr. Trump vowed to undertake the largest deportation effort in the nation’s history, though he skirted questions about whether the sweeps would target undocumented immigrants who had long lived in the country, people who had more recently crossed at the southern border or both. About 11 million undocumented people resided in the United States as of 2022, according to the Pew Research Center, with nearly two-thirds having been in the country for at least a decade.

While deporting millions of people would be all but impossible with current enforcement resources, Mr. Trump has said he would consider stationing American troops at the border with Mexico and working with governors to deploy the National Guard into the interior of the country.

In his victory speech early on Wednesday, Mr. Trump said that voters had handed him “an unprecedented and powerful mandate” to pursue his agenda.

Unlike in 2016, when he won the Electoral College but lost the popular vote, Mr. Trump won both in this election, the first Republican to prevail in the national vote in two decades, after campaigning on harsh immigration policies. And he will enter office with a Supreme Court that counts three of his first-term nominees among the nine justices.

“We’re going to fix our borders, we’re going to fix everything about our country and we’ve made history for a reason tonight, and the reason is going to be just that,” he said on Wednesday.

Lawyers for immigrants said they have been preparing for months for the possibility of large-scale workplace raids, roundups in immigrant enclaves, new restrictions on asylum, the expansion of detention and the termination of programs temporarily shielding some people from deportation.

“The Trump team might think they are ready,” said Camille Mackler, chief executive of Immigration Arc, who sent an SOS email that brought hundreds of lawyers to Kennedy Airport that day in 2017. “But so are we.”

Becca Heller, founder of the International Refugee Assistance Project, which sued the government over the Muslim ban, said that winning the popular vote was not a license to ignore the law. “He can’t act outside the bounds of the Constitution and the Bill of Rights,” she said.

Having battled one Trump administration, she and her allies are ready for a second, Ms. Heller said. “We literally have a blueprint of what they are planning to do, and so we had months and months to figure out how to protect people,” she said.

“Trump has told us what to expect — hate and persecution and concentration camps,” she said, referring to his team’s plans to use military funds to build “vast holding facilities.” “None of us have any illusions about what we are up against this time.”

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