As President Trump signed an executive order that attempts to coerce sanctuary cities into shifting their immigrant-friendly stances, leaders across the nation spoke out against the action, calling it unconstitutional and vowing to protect their diverse communities.

Mr. Trump signed the order Wednesday, along with another that called for the construction of his promised border wall between the United States and Mexico. The sanctuary city order calls for cuts in federal funding for cities that fail to report undocumented immigrants accused of nonviolent crimes to federal authorities seeking to deport them.

The pair of actions could help shift US immigration policy, and have led to outcry from those elected officials who have worked to defend the 11 million people living illegally in the United States. Despite Trump’s strong rhetoric on the campaign trail and immediate actions taken during his first week in office, leaders around the nation say they are prepared to protect the immigrants residing within the limits of some 200 sanctuary cities.

And legal experts have said the order, which threatens to withhold undisclosed amounts of federal funds, may embody an overstep of presidential authority and violate the Constitution.

"There is a huge question to the legality of Trump’s threats. I don’t think Trump unilaterally has the ability to strip federal funding," Paromita Shah, associate director at the National Immigration Project of the National Lawyers Guild, tells The Christian Science Monitor, noting that the president could intend to use the so-far loosely defined order to put pressure on localities that oppose the key tenets of his campaign.

"It’s not clear, as always, if Trump is posturing by deliberately issuing vague commands for cities to do what he wants," she adds.

While the term "sanctuary city" has come into wide use, it doesn’t have a legal definition. Generally, the communities classify themselves as such by refusing to hold undocumented immigrants who commit nonviolent crimes until federal Immigration and Customs Enforcement (ICE) agents can take them into custody.

In cities and towns across the country, the scenarios arise when police arrest someone and charge him or her, taking fingerprints and sending the prints to the FBI and ICE. If federal agents decide to deport the person, local law enforcement officers typically continue to hold him or her for an additional 48 hours and wait for immigration officials to take the defendant into custody, even if the accused has paid bail on a nonviolent charge.

But in sanctuary cities, local officers cannot hold defendants after judges order their release on bail. The policy stems from an argument that detaining people for civil immigration violations after they’re to be released is a violation of the Constitution. Some sanctuary cities bar their law enforcement officials from questioning residents in an attempt to determine their immigration status.

Advocates of the programs argue that they make cities safer by giving undocumented immigrants amnesty as they come forward to report crimes, allowing bridges to be built between minority communities and law enforcement. Others say that shielding immigrants from deportation allows them to apply for business licenses and legally operate small economic ventures that boost the economy. But opponents say the measures keep dangerous criminals in the United States, putting citizens at risk and undermining federal immigration laws.

Trump’s actions are short on specifics, but he again reiterated the idea that the executive order is intended to tackle issues with violent and dangerous undocumented criminals. White House Press Secretary Sean Spicer said the order would confront cities classified as sanctuaries under the Department of Homeland Security regulations and render them ineligible for federal grants.

"They shouldn't be very worried," Trump said of the immigrants, such as those who classify as "dreamers" protected by former President Barack Obama, in an ABC News interview broadcast on Wednesday night. "Where you have great people that are here that have done a good job, they should be far less worried. We'll be coming out with policy on that over the next period of four weeks."

But leaders around the country took the executive order as a call to action, and they may have solid legal backing to maintain their statuses. Cities use federal funding for vital community functions, covering initiatives such as drug and HIV testing, job training, and assisting the elderly community. In December, Mother Jones reported that a slew of cities already indicating their will to resist anti-immigration action could stand to lose as much as 25 percent of their city or police department budgets.

Still, the threat of those losses hasn’t caused leaders to reconsider their stances. Democratic Boston Mayor Marty Walsh called the executive action "the most destructive and un-American threat on America," and offered up Boston City Hall as a safe haven for the city’s undocumented residents.

"If people want to live here, they’ll live here," he said at a press conference Wednesday. "They can use my office. They can use any office in this building."

Others have said they would rely on legal action to fight the directive, which could serve to contradict its anti-terror and crime-fighting goals.

"We think it's very susceptible to legal challenge," New York City Mayor Bill de Blasio (D) told CNN. "If they make an attempt to pull that money, it will be from NYPD, from security funding to fight terrorism. If an attempt is made to do that, we will go to court immediately for an injunction to stop it."

In California, mayors of San Francisco, San Jose, Oakland, and the state legislature have joined together to form a united front against the policy changes.

Trump’s directives could clash with the 10th Amendment, which gives states the rights to create their own legislation in conjunction with federal laws and regulations. It also must stand up to the 1987 Supreme Court ruling South Dakota v. Dole, in which the court found that funding can be withheld only for the specific initiative in question. Experts speculate that Congress, which would have more authority than the president to block funding, will likely be unable to deny grants to a range of programs across cities.

Sanctuary city policies "have been carefully crafted with federal laws in mind," Grisel Ruiz, a staff attorney with the Immigrant Legal Resource Center, tells the Monitor. "They can definitely stand a legal challenge in court."

While aggressive immigration policies have long created a chasm between the ICE and local law enforcement and governments, Trump's use of an executive order and threats to curtail funding represents a bolder push on part of the federal government, Ms. Ruiz adds.

"In terms of someone being this aggressive and going this far out, Trump is really setting a new standard for how aggressive he’s being in trying to coerce cities," she says.

As the specifics of the laws – and their legal viability – continue to be debated, sweeping action on the part of local leaders would likely be premature. Some cities, such as Los Angeles, have set up funds to provide legal services for immigrants, and others have evaluated their local police departments and developed new ways to increase their presence in historically isolated, minority communities. Both of those tactics can serve to protect undocumented immigrants.

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For now, voicing their opposition to the action and reaffirming their commitment to protecting undocumented immigrants in their cities as valuable community members is a strong step for elected officials to take, Ms. Shah says.

"They wouldn’t have passed these policies if they weren’t on firm [legal] footing," she says. "I don’t see anything here that they’re required to change anything that they’re doing. Everything they’ve been doing has been within the confines of the law."