WASHINGTON — Republicans in Congress are taking aim at four cities — often called “sanctuary cities” — over their policies limiting cooperation with immigration enforcement with a hearing this week that comes as President Donald Trump presses ahead with his campaign of mass deportations.

Mayors Michelle Wu of Boston, Brandon Johnson of Chicago, Mike Johnston of Denver and Eric Adams of New York are set to appear Wednesday in front of the House Committee on Oversight and Government Reform.

There’s no strict definition for sanctuary policies or sanctuary cities, but the terms generally describe limited cooperation with Immigration and Customs Enforcement. ICE enforces U.S. immigration laws nationwide but seeks state and local help, particularly for large-scale deportations, requesting that police and sheriffs alert them to people it wants to deport and hold them until federal officers take custody.

But some cities and states say cooperating with ICE means victims of crime and witnesses who aren’t in the U.S. legally won’t come forward. And, to varying degrees, officials argue that they want their localities to be welcoming places for immigrants.

Courts have repeatedly upheld the legality of most sanctuary laws. But Trump administration officials have targeted sanctuary policies right out of the gate in his second term, seeing them as a key impediment to deporting people in the large numbers he wants. The administration has sued Chicago and Illinois as well as New York state over various immigration laws.

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Here’s a look at the immigration policies and migration trends of the cities whose mayors will appear Wednesday:

Boston

The city’s Trust Act generally restricts how much the police can cooperate with ICE, although it does allow some cooperation with a division called Homeland Security Investigations when it comes to such issues as combating human trafficking or drug and weapons trafficking.

The city also must follow a 2017 ruling by the state’s highest court that forbids Massachusetts authorities from holding a person otherwise entitled to release from custody based solely on a federal request.

Those requests, called detainers, typically ask federal, state, local and tribal law enforcement agencies to give at least 48 hours’ notice before suspected immigrants are released from jail — or to hold them for up to 48 hours after they would normally be released so ICE can pick them up. Otherwise, ICE must go out into the community to arrest them.

Trump’s border czar, Tom Homan, hammered Boston’s police commissioner and promised to go to Boston and “bring hell” with him.

That has not sat well in Boston.

Wu, a Democrat up for reelection this year, said that it was “clueless” and “insulting” for Homan to attack the police commissioner and that she wants Boston be a welcoming place for immigrants.

Suffolk County District Attorney Kevin Hayden said handing over defendants makes it harder to gain cooperation from immigrants when investigating crimes.

Chicago

Chicago has some of the country’s strongest immigrant protections, barring cooperation between federal agents and city employees, including police.

The nation’s third-largest city became a sanctuary location in the 1980s and has amped up its policies multiple times since. That includes a 2012 ordinance that bars federal agents from accessing city resources in assisting with immigration operations.

City officials beefed up protections after Trump first took office in 2017 and again in 2020, when it required police to document requests for assistance from federal agents. Illinois also has protections allowing anyone, regardless of immigration status, to get a driver’s license.

Mayor Brandon Johnson, who took office in 2023, inherited a migrant crisis his first year. Short on shelter space, Chicago’s leaders used police station lobbies, airports, parked buses and hotels to house asylum-seekers.

At its height, roughly 15,000 migrants lived in dozens of temporary shelters, although the city has shuttered those as the number of people arriving decreased.

There have been a handful of City Council attempts to weaken Chicago’s sanctuary protections, but they have been unsuccessful in the Democratic-leaning city.

Johnson has defended the policies.

“We welcome all individuals and families who want to work, live and thrive here in our beloved city,” Johnson said in late January.

Denver

Denver became a hub for immigrants coming across the U.S.-Mexico border, with the city seeing arrivals of 43,000 over the last two years.

Mayor Mike Johnston struggled to cobble together the resources to house and feed them, gouging parts of the city budget and pleading with the federal government alongside other mayors for funding, which only partially came.

The spotlight ratcheted up when a video of armed men in an apartment building in the Denver suburb of Aurora went viral, prompting then-candidate Trump to claim that the city of some 400,000 people was overrun by a Venezuelan gang.

The idea was swiftly repudiated by local officials, but the limelight stayed on Colorado.

In early February, heavily armed federal agents in masks and armored vehicles appeared across the metro area. The results of the operation are unclear, and Homan said news media leaks hindered the raids.

The mayor has repeated that he wants Denver to be a welcoming place while drawing the line at those who commit violent crimes.

Last month, Denver Public Schools became the first district to sue the Trump administration over its policy allowing ICE immigration agents in schools.

New York

In New York City, an estimated 231,000 immigrants have arrived since 2022 — at a cost that the city estimates to be more than $7 billion for shelters, food, security and legal support.

Last month, the city sued the Trump administration over $80.6 million in reimbursements for congressionally approved immigrant aid after officials paid the money and then clawed it back.

As in other cities, New York’s immigrant arrivals have fallen dramatically over the past year.

Mayor Eric Adams continues to voice support for policies that ensure all residents have access to social services regardless of immigration status.

But Adams wants to roll back policies prohibiting city employees from working with immigration enforcement, saying he wants to “remove dangerous people from our streets.” He’s also called on the City Council to make changes allowing New York to honor ICE detainer requests to hold people in jails beyond their release dates. And he’s invited ICE agents to work out of the Rikers Island city jail.

Adams’ critics, including those running for mayor, say his desire to work with the White House is an effort to wriggle out of federal corruption charges.

Adams said ahead of the hearing that he hoped for a “civil conversation” with the representatives “around a very complex topic that many cities have inherited.”