America’s rightwing “constitutionalists” should be proud of the left. Last week, state and local authorities mobilized against a family-separation policy handed down from Washington DC, and in doing so, took a page from the playbook of anti-government conservatives.

Now, having tasted victory, local officials from Atlanta to New York are not backing down from their tactics. After Donald Trump ended his policy on taking immigrant children from their parents and detaining them separately, mayors and governors continued to speak out. On Thursday, mayors from more than a dozen cities gathered outside a detention center on the border to demand that the roughly 2,000 families that have already been separated be reunited.

Criticism from members of the president’s own party has carried extra weight. Over the weekend, Ohio governor John Kasich, for example, called on Republicans to fix the “humanitarian crisis” at the border.

Refusing to cooperate

At least three other governors, including one Republican, recalled their National Guard troops from the border to protest the Trump policy. Though largely symbolic—troops have so far done little to curb immigration, according to reports—the withdrawals represent a big shift in attitude. When Trump requested states to contribute extra boots on the ground in April, even governor Jerry Brown from liberal California agreed to help.

Others are distancing themselves from the federal government in other ways. On Wednesday, Atlanta mayor Keisha Lance Bottoms refused to take in federal detainees at the city jail, which has a contract to temporarily house them. On Thursday, she said she’s considering canceling the contract altogether. El Paso County sheriff Richard Wiles said he will not allow any of his officers to work off-duty at the “tent city” erected near the border to temporarily house children separated from their parents.

Ilya Somin, a law professor at George Mason University, says the non-cooperation principle could extend to other areas “as leverage to try to force the federal government to give in” on its restrictive immigration policies.Federal agencies rely on the help of local officials in other arenas, for example, combating drugs.

Getting tough on federal contractors

In Houston, mayor Sylvester Turner suggested city officials could stretch out the permitting process for a government contractor’s plans to convert a warehouse into a “tender age shelter.” “I do not want to be an enabler,” he said at a press conference last week. “If we don’t speak, if we don’t say no, then these types of policies will continue.”

The mayor also urged the contractor, Southwest Key Programs, not to go ahead with its plans, and the state health department not to license it.

Texas is unlikely to refuse a license for an immigrant detention facility—the state has committed to cooperating with federal authorities on immigration. But the strategy could work in other states. California, for example, already has a law that allows state inspectors to check immigrant detention facilities for violations. Federal lawyers have sued the state over the practice, which they say obstructs the enforcement of immigration laws. The judge hearing the case, however, on Wednesday suggested he doesn’t fully agree.

“Where’s the evidence of it being burdensome in any way?” he asked federal lawyers during a hearing.

Suing the Trump administration

New York governor Andrew Cuomo is pressing on with his plans to sue the Trump administration over the family separation policy. The new executive order, he said in an interview with CNN, is more of a press release than a legal document. This is how Cuomo justified the lawsuit in a New York Times editorial that was published after Trump’s announcement Wednesday.

Holding the children apart from their families is a violation of the constitutional rights of parents to care for, maintain custody of and communicate with their children. These parents are afforded the fundamental right to family integrity under the United States Constitution and under the New York State Constitution. By systematically separating parents from their children, this administration has shown complete disregard for parental rights.

This is a novel argument in the category of immigration-related lawsuits filed by states against the federal government. In the past, states have claimed that a federal policy caused them harm. In 2016, for example, Texas successfully argued that an Obama administration’s program to protect undocumented immigrants with American children would force the state to spend millions of dollars. Under the program, known as Deferred Action for Parents of Americans or DAPA, the immigrants would be eligible to get drivers licenses, which Texas said would cost it $200 a piece to process. More recently, Hawaii argued that Trump’s travel ban hurt the state by dissuading tourists and foreign students from coming.

In contrast, Cuomo is saying that the federal government is hurting the state’s residents, not the state itself. Several dozen immigrant children are being held in shelters around New York.

Though it’s unclear whether that argument would pass muster in court, it would definitely increase the federal government’s legal costs and its lawyers’ workload.

Double-edged sword

Of course, the local fight against the federal government could backfire. The Trump administration has already tried to retaliate by removing federal aid from jurisdictions that refuse to hand over immigrants without a court warrant. Its efforts have been blocked in court so far—two federal judges have ruled the government doesn’t have the right to force cities to cooperate on immigration. (In the cases before the courts, those cities are Chicago and Philadelphia.)

But the administration could come up with other ways to strike back. Federal immigration agents told a federal judge that the purpose of a raid they conducted in Austin last year was to get back at the city for its local sanctuary policies. More than 50 immigrants were picked up that day.

And in the future, the same legal strategies that liberal state and local politicians are testing in court could later be used by their opponents to achieve the opposite goal. They should know: they’ve already repurposed the “harm” arguments Texas used against Obama’s DAPA to attack Trump’s travel ban.

“It’s a tricky thing to take on the federal government,” said Cristina Rodríguez, a law professor at Yale University.