Donald Trump’s tough stance on illegal immigration drove him to the front of the Republican primary pack, and few of his barbs were more effective than his attacks on “sanctuary cities” where, he suggested, criminal aliens were basically allowed to roam free and assault American citizens.

The stories about victims, highlighted by Trump, underscored a sense that Americans were under siege and concern about US immigration priorities resonated deeply with the electorate.

This week, Mr. Trump said he will “cancel” billions of federal dollars to some 39 US sanctuary cities until local police start cooperating with federal immigration officers to find, detain and deport undocumented people. Though they differ in shape and form, sanctuary policies in essence order police to not ask about people's immigration status in the normal course of business, and as long as any charges were misdemeanors. But five US states have "papers, please" laws that require police to check the immigration status of suspects.

It’s far from clear whether a Trump administration could break the immigration impasse only made starker by the fact that the US capital, Washington, D.C., officially shelters undocumented immigrants. This week, at least seven big-city mayors declared they would not abandon their sanctuary policies, despite Trump’s de-funding threat.

This is an issue that will likely test both Constitutional law and Congressional will – and could become a critical initial gauge of how big a fight the Trump administration is willing to pick in order to fulfill a key campaign promise. Bottom line, however, is that “there’s a great deal the Trump administration could do to carry out the threat,” as George Washington University constitutional law professor Jonathan Turley tells The Wall Street Journal.

Born in the 1980s with the support of faith groups whose members faced arrest for harboring people they saw as refugees along the US-Mexico border, the sanctuary city movement is today led more by local police than priests. Los Angeles Police Department Chief Michael Beck, whose city passed the country's first sanctuary city ordinance, called it a matter of “principle” over “money.”

As the Trump agenda on immigration takes shape, the attack on sanctuary cities presents an unusual dynamic: A law-and-order Republican administration potentially invoking federal power to coerce local cops to bend to Washington's will.

Trump, legal groups, and conservative lawmakers argue that what’s really happening is that serious criminals are slipping through the cracks and ending up back on the streets.

For their part, law enforcement officials in many sanctuary cities say their policies protect the public’s safety. As Washington, D.C., Mayor Muriel Bowser said in a statement: “We are a sanctuary city because we know that our neighborhoods are safer and stronger when no one is afraid to call on our government for help, and when our police can focus on protecting and serving."

US courts, too, have taken a skeptical view of the assertion of federal power over local police departments. The Supreme Court in a 1997 gun rights case ruled that local police can’t be treated as de facto federal agents. Many Republicans applauded that ruling as a major states’ rights victory.

But more recent rulings may be more consequential to US police chiefs. In 2014 a federal judge ruled that the Clackamas County, Ore., Sheriff’s Department had violated a woman’s Fourth Amendment right against unlawful detention. How? It had kept her for over 19 hours without being arrested, under a federal immigration detention hold. Suddenly, small police departments all across the region were open to lawsuits were they to hold people for too long without probable cause, even if it was as a favor to the feds.

That and other rulings have affirmed that federal immigration authorities “don't have the power to directly force states and localities to implement policies and procedures that the states and localities themselves have not decided to do," Phil Torrey, a Harvard Law School lecturer, tells US News & World Report.

On top of that, Congress has tried and failed to shut off funds to sanctuary cities before, most recently last year. Chicago Mayor Rahm Emanuel told reporters that he doubts Trump is serious about targeting "every major city in the United States."

After all, "This is not a bunch of left-wing radicals ... these are sheriffs and police chiefs,” Pastor John Fife, one of the first to take part in the sanctuary movement, told MSNBC recently.

At the same time, Trump definitely has options.

For one, the Justice Department found in July that some sanctuary cities could be violating federal laws that make it illegal to withhold immigration information from Immigration and Customs Enforcement (ICE). That analysis, however, didn’t touch on the Fourth Amendment complications of detaining people without warrants.

And whether Congress is willing to defund sanctuary cities or not, Trump could as president redirect discretionary grants to cities – for some, perhaps worth enough to force their hand. Moreover, Trump could immediately reverse a 2012 Obama order that focused agents mostly on violent offenders. In its place, he could broaden the focus of ICE to target people who have been convicted, or even just accused, of minor crimes, including traffic infractions. He could also ask Congress to beef up the ranks of federal immigration agents, which now number around 14,000.

More dramatically, a Trump Justice Department could conceivably arrest police chiefs under laws against harboring fugitives from the law. " "It's never been used that way, but it's something that authorities should look at if it comes to that,” Jennifer Vaughn of the anti-immigration Center for Immigration Studies told US News & World Report.

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Such debate may in part miss Trump's real goal. At the core of Trump’s stated strategy, after all, is making beefed-up deportation a hallmark of his presidency. In that, he will try to match a record 2.5 million deportations racked up by the Obama administration -- though Trump told CBS News that he wanted that number deported almost immediately. But Trump has also suggested he's open for debate as to what should happen to the perhaps 8 or 9 million remaining undocumented immigrants who are not seen as posing a threat to Americans.

"After the border is secure and after everything gets normalized, we're going to make a determination on the people that they're talking about who are terrific people," Trump said.