Should President Trump follow through on a proposal to release migrants in U.S. “sanctuary cities,” it would be a major departure from the way federal agencies are handling detainees. It could also be prohibitively costly and make it more difficult to deport migrants once they reach those cities.

The plan — which Trump tweeted Friday is under “strong consideration” — would have the Department of Homeland Security moving migrants from detention centers to cities scattered across the country in vans, buses and airplanes. It would require a massive investment in transportation infrastructure, something that Immigration and Customs Enforcement officials told the White House would be “an unnecessary operational burden.”

It also would mean placing those detainees in cities that limit their cooperation with federal immigration enforcement, meaning it could be very difficult to arrest them again.

During the recent surge of Central American families crossing into the United States, most were apprehended at or near the southern border with Mexico. With a deficit of detention beds, the U.S. government mainly releases the families to shelters or bus depots. Detainees are sometimes released directly to the streets of border towns, allowing immigration authorities to focus staffing and funding on deportations and criminal operations.

Trump’s proposal, which government officials said is aimed at punishing Democratic strongholds for their positions on immigration policy, calls for sending the detainees to sanctuary cities, where they can live without fear of local authorities reporting them to federal immigration officials. There are hundreds of sanctuary jurisdictions nationwide, ranging from tiny rural counties to New York City and the entire state of California.

President Trump told attorneys general on March 4 to make sure their “states and cities are fully cooperating with the Department of Homeland Security." (The Washington Post)

The idea, DHS officials said, seemed predicated on the belief that an influx of migrants would be a burden to sanctuary cities. Trump has long maintained that killers, rapists and drug dealers are streaming across the border and that releasing migrants into U.S. society is a security risk. In fact, studies show immigrants are less likely to commit crimes than native-born citizens.

Mayors of such cities condemned the White House plan on Friday, with most dismissing it as an unrealistic political stunt. Some already have waged successful legal battles against the Trump administration’s threat two years ago to slash federal funding to sanctuary cities.

Libby Schaaf, the mayor of Oakland, Calif., called the plan “an outrageous abuse of power — using human beings to settle political scores.” San Francisco Mayor London Breed said it “is just another in a long line of scare tactics and half-baked ideas.”

Mayor Joseph A. Curtatone of Somerville, Mass.,which has a population of 81,000, said he would welcome any immigrants the government wants to send his way.

“Fine by me,” he said on Twitter, firing back at Trump. “But does he realize that the moment after people get ‘placed’ they’ll start moving to wherever they want to go? Every city has an open border.”

Homeland Security prefers to detain immigrants until they are eligible for deportation, but officials are releasing tens of thousands every year because of mass migration from Central America, rising numbers of families, limited detention space and legal restrictions on how long the government can detain children.

U.S. Customs and Border Protection apprehended 103,000 migrants last month — double the number in March 2018 — including nearly 60,000 family members.

CBP typically transfers migrants to ICE for detention, though this year holding cells grew so crowded that border agents started releasing some families at the border. ICE can also release migrants on bond or ankle-monitoring devices after verifying their future address and handing them a notice to appear in immigration court. Unaccompanied migrants are sent to Health and Human Services shelters, where case workers find a parent or guardian for them to live with in the United States.

Congress has allocated billions of dollars for this system, and none of it involves transporting immigrants to sanctuary cities — which some say makes the president’s plan illegal.

“It makes no sense,” said John Sandweg, an acting ICE director in 2013 and 2014 in the Obama administration, adding that it would violate federal law by diverting money “for political purposes.”

“At a time like this, when ICE is just overwhelmed by the number of Central Americans arriving, having to divert further resources to send a political message is outrageous,” he said.

Sandweg said the government “would pay big money” for the White House’s plan to deliver migrants to sanctuary cities. In addition to transportation costs, officials would have to assign immigration agents to escort them to their destinations. Currently, migrants usually buy their own bus or airline tickets.

“It’s ludicrous,” Sandweg said. “It’s meddling in operations at an extreme level.”

Matthew Albence, ICE’s acting deputy director, questioned the proposal in an email to the White House in November after it was first raised as a possibility, saying that arranging for transportation would strain the department and weaken its enforcement efforts.

“As a result of the influx at the border and the record number of aliens in detention, we have already had to decrease our interior operational footprint to manage these cases, resulting in less officers out on the streets making arrests of criminal aliens, public safety threats, fugitives, and other immigration violators,” Albence wrote in an email reviewed by The Washington Post. “Not sure how paying to transport aliens to another location to release them — when they can be released on the spot — is a justified expenditure.”

After heeding Albence’s advice not to pursue the idea, the White House went back to DHS in February to try again. Legal advisers rejected it.

Jessica Vaughan, director of policy studies for the Center for Immigration Studies, which favors less immigration, said the plan would give migrants a free ride to their destinations. Because sanctuary cities often refuse to turn over migrants arrested for crimes to ICE, sending them there could make it more difficult to apprehend for deportation later, she said.

Vaughan said White House officials who are new to immigration policy have likely overstepped in this case.

“There are a lot of immigration policy amateurs in senior positions at the White House, and some of them should stay in their lane — which is not immigration,” she said.

On the campaign trail in 2016, Trump said blocking funding for sanctuary cities would be a top priority, saying at the time: “Cities that refuse to cooperate with federal authorities will not receive taxpayer dollars, and we will work with Congress to pass legislation to protect those jurisdictions that do assist federal authorities.”

But Congress has not passed any such legislation, and Trump’s other efforts to stem migration have faced legal challenges. At least seven federal courts have blocked the Trump administration from broadly cutting off funds to sanctuary jurisdictions.

Vaughan said the Trump administration has conditioned some Justice Department crime-fighting grants on local cooperation with immigration enforcement. But generally that is limited to a provision in federal law that says local governments cannot prohibit communication between police and federal immigration agents.

The law does not require localities to detain immigrants after police have arrested them for an unrelated crime, but ICE can pick them up when a judge releases them from their criminal cases.

After Trump took office, sanctuary jurisdictions were initially fearful that he would restrict their federal funding for school lunches, fuel aid and other essential programs. But those fears faded as they prevailed in court.

Hundreds of localities have since strengthened their sanctuary policies, according to the San Francisco-based Immigrant Legal Resource Center. California passed a slate of new laws and the highest court in Massachusetts said local law enforcement cannot detain someone based solely on an immigration detainer.

Curtatone, Somerville’s mayor, said that the city is “always going to be a sanctuary and welcoming city for all” and that an influx of immigrants wouldn’t change much for cities such as his.

“Somerville has experienced a continuous wave of immigration now for well over a century of Europeans and those from the Caribbean and Central and South America,” he said in a telephone interview. “We speak more than 52 languages in our neighborhoods and our schools. We embrace it.”

Fred Barbash contributed to this report.