Such an action could have far-reaching impacts across the U.S., potentially even in places that aren’t particularly friendly to noncitizens.

Two previous efforts by Trump to cut off some funding for sanctuary jurisdictions were shut down by courts.

Trump unveiled the concept this time late in a speech Tuesday at the Detroit Economic Club, without offering specifics.

“Starting Feb. 1, we’re not making any payments to sanctuary cities or states having sanctuary cities, because they do everything possible to protect criminals at the expense of American citizens and it breeds fraud and crime and all of the other problems that come,” he said. “So we’re not making any payment to anybody that supports sanctuary cities.”

Back in Washington, Trump was asked by reporters what kind of funding would be affected on Feb 1: “You’ll see,” he said. “It’ll be significant.”

There is no strict definition for sanctuary policies or sanctuary cities, but the terms generally describe limited cooperation with Immigration and Customs Enforcement.

Courts have rejected the idea before

In an executive orders last year, the president directed federal officials to withhold money from sanctuary jurisdictions that seek to shield people in the country illegally from deportation.

A California-based federal judge struck it down despite government lawyers saying it was too early to stop the plan when no action had been taken and no specific conditions had been laid out.

In Trump’s first term in office, in 2017, courts struck down his effort to cut funding to the cities.

Some of the details are tricky

The Justice Department last year published a list of three dozen states, cities and counties that it considers to be sanctuary jurisdictions.

The list is overwhelmingly made up of places where the governments are controlled by Democrats, including the states of California, Connecticut and New York, cities such as Boston and New York and counties including Baltimore County, Maryland, and Cook County, Illinois.

That list replaced an earlier, longer one that was met with pushback from officials who said it wasn’t clear why their jurisdictions were on it.

The administration has been threatening funding in specific places

The federal government has moved to halt funding for a variety of programs in recent weeks and is already facing legal challenges.

The U.S. Department of Agriculture has warned states that have refused to provide data on recipients of Supplemental Nutrition Assistance Program money that they’ll be docked administrative funds. A court fight over the request for information was already under way before the threat came. Money hasn’t been stopped yet.

The U.S. Department of Health and Social Services said last week that it was halting money from five Democratic-led states for daycare subsidies and other aid to low-income families with children over unspecified suspicions about fraud. A court put that on hold

The administration has tried to use additional financial pressure against Minnesota, a state where it has also sent a wave of federal officers in an immigration crackdown. The Agriculture Department has said it’s freezing funding in the state — but without laying out many details.

The Centers for Medicare and Medicaid Services also told Minnesota last week that it intends to withhold $515 million every three months from 14 Medicaid programs that were deemed “high risk” after rejecting a corrective action plan it demanded because of fraud allegations. The amount is equivalent to one-fourth of the federal money for those programs. State officials said Tuesday that they’re appealing.