These grants make up a relatively small part of the federal budget and are not a substantial source of revenue for larger cities. Federal funds made up 10 percent of New York City’s $80.5 billion budget in 2015, and $60 million in justice grants is just .75 percent of the city’s grant revenue.

[After years on the outside, foes of legal immigration find a louder voice with Trump’s election]

In 2015, the House passed a bill to prevent sanctuary cities from receiving one federal grant directly related to immigration. The State Criminal Alien Assistance Program reimburses part of the cost of housing inmates during the second phase of deportation proceedings. The grants do not reimburse jails for holding inmates on detainer requests — that’s on their own dime, according to a report by the Immigrant Legal Resource Center.

But a Washington Post analysis found that many counties with sanctuary policies get little or no money from this program. The Department of Justice paid $165 million for the grants last year, with $18 million going to jurisdictions with policies of not cooperating with ICE.

Cutting funding isn’t the only way for the Trump administration to get places with sanctuary policies to help with deportation. Vaughan said another option is seeking an injunction in federal court to block specific policies, especially in jurisdictions that “will not cooperate in any way with ICE.” She said the worst offenders are Cook County, Ill., (home to Chicago), King County, Wash., (Seattle), and three counties around San Francisco.

These and other cities and counties have sanctuary policies that go beyond rejecting detainer requests. Some jurisdictions instruct police to not ask about immigration status; offer municipal identification cards to illegal immigrants; or offer interpreters in city offices. D.C. recently set up a legal defense fund for illegal immigrants. Deportation proceedings are held in civil court instead of criminal court, so defendants don’t have access to a public defender.

“I’m curious to see exactly what he does in terms of this defunding of sanctuary cities because it doesn’t make sense from a legal standpoint or a political standpoint,” Torrey said.

Trump’s order also asked DHS to identify localities that don’t comply with detention requests, and, once per week, to publish a “comprehensive list of criminal actions committed by aliens.”

[President Trump is likely to keep a close eye on this brewing sanctuary city battle in Texas]