If re-elected, Donald Trump has announced mass deportations of people who do not have legal residency in the United States.

While his campaign team gave various answers to the question of how many jobs could be cut, his vice presidential candidate JD Vance gave only one number in an interview with ABC News this week.

“Let’s start with a million,” he said. “That’s where Kamala Harris failed. And then we can go from there.”

But while this policy is a central plank of Trump’s campaign platform—his rallies feature signs reading “Mass Deportations Now!”—experts warn that expelling so many people poses significant legal and practical challenges.

Immigration advocates warn that deportations would also have human consequences: They would tear families apart and lead to raids in communities and workplaces across the United States.

Current figures from the Department of Homeland Security and Pew Research estimate that there are currently about 11 million undocumented immigrants living in the United States. This number has remained relatively stable since 2005.

Most of them are long-term residents – almost four-fifths have lived in the country for more than a decade.

Immigrants who are in the country without legal status are entitled to due process, including a court hearing before they are deported. A sharp increase in deportations would likely entail a significant expansion of the immigration court system, which is plagued by backlogs.

Most immigrants already in the country enter the deportation system not through encounters with Immigration and Customs Enforcement (ICE) officials, but through local law enforcement.

However, many of the country’s largest cities and counties have passed laws limiting local police cooperation with ICE.

The Trump team has promised to take action against these “sanctuary cities.” But the multitude of local, state and federal laws in the United States complicates the picture even further.

Kathleen Bush-Joseph, a policy analyst at the Migration Policy Institute (MPI) in Washington, said cooperation between ICE and local authorities is a “critical” aspect of any mass deportation program.

“It’s much easier for Ice to pick someone up from jail when local law enforcement cooperates rather than having to search for the person,” she said.

As an example, Ms Bush-Joseph pointed to a statement from the Broward and Palm Beach county sheriff’s offices in Florida in early August, which said they would not use deputies to assist in any mass deportation plans.

“There are many others who would not go along with Trump’s mass deportation plan,” she said. “That makes it so much more difficult.”

Moreover, any mass deportation program would almost immediately face a flood of lawsuits from immigration and human rights activists.

However, a 2022 Supreme Court ruling means courts cannot issue injunctions to enforce immigration policies – meaning they will remain in place even if challenges work their way through the legal system.

Even if the US government succeeds in legally pushing forward its plans for mass deportations, the authorities would still face enormous logistical challenges.

During the Biden administration, deportation efforts have focused on migrants recently apprehended at the border. Migrants being deported from the interior of the United States, from areas not near the border, are overwhelmingly people with criminal histories or those deemed to be a threat to national security.

Controversial raids on construction sites conducted during the Trump administration were suspended in 2021.

The number of deportations of people arrested in the U.S. interior – as opposed to those at the border – has remained below 100,000 for a decade, after peaking at over 230,000 in the first years of the Obama administration.

“To increase that amount to a million in a single year would require enormous resources that probably don’t exist,” Aaron Reichlin-Melnick, policy director at the American Immigration Council, told the BBC.

For one thing, experts doubt that Ice’s 20,000 agents and assistants would be enough to find and track down even a fraction of the people promised by Trump’s campaign team.

Mr Reichlin-Melnick added that the deportation process is lengthy and complicated and only begins with the identification and arrest of an undocumented migrant.

The detainees would then have to be housed or placed in an “alternative to detention” program before being brought before an immigration judge, resulting in backlogs lasting years.

Only then will the prisoners be deported from the United States, a process that requires the diplomatic cooperation of the host country.

“In each of these areas, Ice simply does not have the capacity to handle millions of people,” Reichlin-Melnick said.

Trump has said he will involve the National Guard or other U.S. military forces to assist with the deportations.

Historically, the U.S. military’s role in immigration issues has been limited to support functions at the U.S.-Mexico border.

Other than the use of the military and the “deployment of local law enforcement,” Trump has given little concrete information on how such a mass deportation plan could be implemented.

In an interview with Time magazine earlier this year, the former president said only that he would “not rule out” building new detention centers for migrants and that he would support granting police immunity from prosecution by “liberal or progressive groups.”

He added that there could also be incentives for state and local police departments to participate and that those who did not “would not share in the riches.”

“We have to do this,” he said. “This is not a sustainable problem for our country.”

The BBC has asked the Trump team for further comment.

Eric Ruark, research director at NumbersUSA, an organization that advocates for stricter immigration controls, said any deportation program from within the country would only be effective if it was accompanied by increased border security.

“That has to be the priority. If that’s not the case, you’re going to make very little progress inland,” he said. “That’s what makes people come.”

In addition, Mr Ruark said that tougher action was also needed against companies that employ undocumented immigrants.

“They’re coming for work,” he said. “And they’re getting those jobs because domestic security checks have basically been dismantled.”

Experts estimate that the total cost of a million or more deportations would amount to tens or even hundreds of billions of dollars.

ICE’s budget for transportation and deportation was $420 million (£327 million) in 2023. That year, the agency deported just over 140,000 people.

Thousands of immigrants are being held while awaiting court hearings or deportation, and the Trump camp plans to build large camps to house them all.

In addition, the number of repatriation flights would have to be increased dramatically. Military aircraft might be needed to supplement current capacities.

Even a small expansion in one of these areas could result in significant costs.

“Even a minor change is in the tens or hundreds of millions,” said Reichlin-Melnick. “A significant change is in the tens or hundreds of millions.”

These costs would be in addition to the costs of other border protection measures promised by Trump: continuing work on a wall on the southern border of the United States, a naval blockade to prevent the import of fentanyl into the country, and the deployment of thousands of troops to the border.

Adam Isacson, a migration and border expert at the Washington Office for Latin America, said that “nightmare images” of mass deportations from a possible Trump administration could also prove politically costly from a public relations perspective.

“Every community in the United States would see people they know and love being put on buses,” Mr. Isacson said.

“On television you see very painful images of children and families crying,” he added. “All of this is incredibly bad press. It’s family separation, only on steroids.”

During the four years of the previous Trump administration, approximately 1.5 million people were deported from both the border and the interior of the United States.

The Biden administration – which has deported about 1.1 million people through February 2024 – is on track to reach that number, statistics show.

During the Obama administration’s two terms – when Biden was vice president – more than three million people were deported, prompting some immigration reform advocates to dub Barack Obama “deporter in chief.”

The only historical comparison with a mass deportation program was in 1954, when up to 1.3 million people were deported as part of Operation Wetback. The name of Operation Wetback comes from a derogatory slur against Mexicans that was widely used at the time.

However, this figure is disputed by historians.

The program under President Dwight Eisenhower faced considerable public opposition—in part because some U.S. citizens were also deported—and a lack of funding, and was largely discontinued by 1955.

Immigration experts say the earlier operation is not comparable to a modern mass deportation program because of its focus on Mexican citizens and lack of due process.

“Those deported in the 1950s were single Mexicans,” said Kathleen Bush-Joseph of the MPI.

“Today, the vast majority of people traveling between ports of entry come from countries outside Mexico or northern Central America. This makes their repatriation much more difficult,” she added.

“These are not comparable situations.”