Immigration and Customs Enforcement––the federal agency responsible for deportations––just got a little more powerful.

In a surprise appearance at the White House press briefing, Attorney General Jeff Sessions announced that the Justice Department is taking steps to implement one of President Donald Trump’s core campaign promises by withholding some federal funds from so-called sanctuary cities. The move will push law enforcement officers in towns, counties and cities around the country to cooperate more with ICE, even if they think that cooperation could undercut their relationships with immigrant communities.

“I strongly urge our nation’s states and cities and counties to consider carefully the harm they are doing to their citizens by refusing to enforce immigration laws and to rethink these policies,” Sessions said. “Such policies make their cities and states less safe. Public safety as well as national security are at stake.”

“Sanctuary cities” isn’t a legal term with a clear definition, but the phrase generally refers to jurisdictions whose police don’t always automatically cooperate with ICE as it tries to track down undocumented immigrants to be deported. From the first days of his campaign, Trump promised to withhold federal grant money that went to these jurisdictions (there are upwards of 600 of them, according to the National Immigration Law Center). And now he’s doing just that.

Sessions announced that unless jurisdictions “certify compliance” with a section of the law––8 U.S. Code 1373, to be precise––they could lose federal grant money. Sessions also said that he would take any steps possible to “claw back” grants already disbursed to jurisdictions that violate that section of the law.

Experts disagree about whether or not jurisdictions break federal law when they ignore ICE’s requests to hold undocumented immigrants in jail so the agency can pick them up and deport them. Immigrants’ rights advocates and the ACLU say sanctuary jurisdictions aren’t violating any federal laws.

“Few, if any, localities are in violation of 1373,” said Domenic Powell of the ACLU, referring to the section of the U.S. Code that Sessions cited. “There’s no way to pretend that 1373 requires blind compliance to detainers.”

It’s clearly not a view the attorney general shares. In his White House remarks, he noted that the Justice Department will give out more than $4.1 billion in grants this fiscal year to help jurisdictions with law enforcement and criminal justice issues. And analysis from the liberal Center for American Progress found that in the last fiscal year, so-called sanctuary jurisdictions received more than $870 million from the feds.

The federal government has extraordinary leverage over jurisdictions that rely on that grant money for law enforcement. Travis County, Texas––where Austin is located––recently lost $1.5 million in state money because its sheriff, Sally Hernandez, announced the county would stop fully cooperating with ICE. The loss could force the county to cut programs that support veterans with PTSD and parents with drug addictions, unless the sheriff there decides to start working with ICE again. State Rep. Eddie Rodriguez, a Democrat who supports the sheriff’s policy, told The Daily Beast before Sessions’ announcement that increased financial pressure could put significant pressure on Hernandez.

“At some point, you hit a point where the cuts are so much, it becomes so great that if you’re the sheriff, you have to have that discussion—you have to figure out what you’re going to do,” he said. “And there probably is a point where the purse strings are such that it’s going to be hard for her not to change her policy.”

That’s exactly what Sessions is pushing for.