As a candidate, Trump made immigration one of his highest-profile issues, saying he’d create a “special deportation task force” and build a “beautiful” wall along a major part of the southern border with Mexico. But immigration experts say those promises, though popular among his base, pose major complications as Trump prepares for office.

Mass-scale deportations would upend parts of the economy, particularly in the agricultural sector, while dividing families and costing the government billions of dollars at a time of constrained finances. The wall would make a political statement but would do little to stop a major portion of new arrivals — those seeking asylum and overstaying their visas. It would also face enormous obstacles in being built, experts say, including environment and engineering problems and the huge topographical challenges of the border itself.

Yet even with the potential difficulties, Trump will have the administrative tools to massively scale back the Obama administration’s efforts to shield millions of immigrants from deportation, former federal officials say. With the stroke of a pen, for example, Trump could reverse a program that has protected hundreds of thousands of people brought to the United States illegally as children — something he has vowed to do.

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The new president could also change the priorities of the Department of Homeland Security, exposing even more of the nation’s estimated 11 million undocumented immigrants to sudden deportation, including longtime residents with families.

“I would expect a sea change in the enforcement landscape,’’ said John Sandweg, a former acting director of U.S. Immigration and Customs Enforcement, a part of DHS that would oversee heightened deportations. “The reforms made in immigration enforcement over the last eight years have all been administrative, not statutory. Trump has tremendous flexibility to reverse course on all those policies.”

Though Trump’s immigration policies proved popular with his base of working-class white voters, critics say they are at odds with a moment in which the United States' illegal immigration population is already tapering off. The number of people trying to cross the country's southwest border has declined significantly over the past decade, and more Mexicans are returning home than entering.

Trump has also said virtually nothing about two other major immigration issues: the surge of Central Americans fleeing violence and seeking asylum in the United States, and the resulting backlog in the country's overworked immigration court system. There, many new arrivals wait for years to have their cases heard. Trump also hasn’t taken a clear position on the detention of asylum-seekers, including families, a practice that expanded drastically under President Obama.

The estimated 11 million people living in the United States without documentation are here either because they entered the country illegally or overstayed their visas. Trump, in an August speech, said that those people’s lives would change rapidly during a Trump presidency. While the Democratic candidate Hillary Clinton had proposed a path to citizenship for some of those people, Trump has said they have only one way to gain legal status: leave and return with a visa, something that could require years of waiting.

Trump initially indicated that any undocumented immigrant was vulnerable to deportation and then said that he would first target those with criminal records. At first glance, prioritizing the deportation of criminals parallels Obama’s policy. But Trump would dramatically expand the pace. Trump said that 2 million unauthorized immigrants have criminal records. (The nonpartisan Migration Policy Institute puts the number at 820,000.)

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“Day One, my first hour in office, those people are gone,” Trump said. “And you can call it deported if you want. The press doesn't like that term. You can call it whatever the hell you want. They're gone.”

The president has wide latitude to issue directives to ICE, as Obama did in 2014, when he asked the agency to focus its efforts on those who posed threats or had been convicted of crimes and de-emphasize targeting of families living peacefully. Since then, the overall number of deportations has declined. But a greater proportion of those removed are criminals.

“The president can do a lot simply by changing the imperatives of the bureaucracy,” said William Stock, president of the American Immigration Lawyers Association. “What will be interesting is — right now, most of the 11 million [undocumented] immigrants have been here for well more than 5 years. The law says they are entitled to a hearing to find out if they are entitled to some form of relief. The question will be, how far will the agencies be encouraged to shortcut due process?”

Trump has said his deportations will be carried out in a “very humane” way, but he has also spoken admiringly of the United States' 1954 operation, authorized by Dwight Eisenhower, in which hundreds of thousands were rounded up and taken back to Mexico in overcrowded and sometimes-deadly trips. Trump said in a CBS “60 Minutes” interview with Scott Pelley that the operation was “very successfully done.”

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“I know it doesn’t sound nice, but not everything is nice,” Trump said.

“It doesn’t sound practical,” Pelley said, who added that “there is something called civil rights.”

“There is also called, ‘We have a country,’ ” Trump said.

Trump has said he wants to triple the number of ICE agents. Even so, rounding up millions of people, criminals or not, would require monumental expense and potentially expose Americans to all kinds of disruptions. Agents can’t simply break down the doors of homes, looking for illegal immigrants. And so there would have to be a surge in raids at farms and factories. New checkpoints might be established along roads. For those who are apprehended and awaiting deportation, the United States would also need to build new detention facilities — or pay private companies to do so.

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Trump has also pledged to clamp down on so-called sanctuary cities, places in which local authorities decide not to proactively ask immigrants for paperwork, even if they believe they’re undocumented. Trump could pressure these cities by cutting off or reducing federal funding, a step for which he’d need support from Congress.

Tripling ICE’s immigration-agent core, which currently numbers about 7,000, would be “incredibly expensive” and unlikely to be approved by a Senate run by Republicans but with a large Democratic minority, Sandweg said.

“Think about all the downstream consequences of that,’’ he said.

“You need more officers around the country. You need office space, you need all those guns, uniforms, slots at the academy for training. We’re talking about possibly close to $1 billion a year in increased costs.’’

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At immediate risk, former officials and immigrant advocates say, is Obama’s Deferred Action for Childhood Arrivals program, known as DACA. The 2012 initiative has given temporary protection from deportation to hundreds of thousands of people who arrived in the United States as children. Those immigrants say the program has helped them emerge from the shadows, making possible a work permit and a Social Security number.

But DACA is not a law; it was not even created by an executive order. It was established by a policy memo sent out by former homeland security secretary Janet Napolitano. Trump, if he follows through on his promise, could simply rescind it.

Which would mean that anybody who gave the government their information and location to participate in the program could be much more easily found by federal deportation agents.

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“The elimination of DACA is a serious threat and a serious concern,’’ said Sandweg, who helped design the program as a senior homeland security official in the Obama administration. “It could be done administratively, and you could absolutely use the information provided for targeting purposes.’’

Then there is the wall. The United States has already spent billions in recent years fencing off about one-third of its southern border. But Trump’s signature proposal calls for finishing the job, trucking in steel and concrete while also entering into likely courtroom battles with ranchers who don’t want to give up their land.