Advisers to President-elect Donald Trump are thinking up ways to cut federal funding to cities that do not cooperate with his plans to root out and deport immigrants lacking permanent legal status, according to a Tuesday report in The Washington Post.

The Democratic leaders of some states and cities, like Chicago and Denver, have pledged not to lift a finger to assist with Trump’s effort, preventing local law enforcement from using their resources to help Immigration and Customs Enforcement.

The Post cited three unnamed individuals familiar with the Trump team’s discussions.

Mass deportation was one of Trump’s biggest promises on the campaign trail, although it is unclear how swiftly he will be able to carry it out as president, given the legal and logistical hurdles involved.

Cities receive federal funding like grants for a variety of services, such as public transportation, affordable housing programs and crime reduction efforts. In Chicago, officials are rushing to finalize a federal grant before Trump takes office to lock down $1.9 billion for an extension on the city’s train service that is set to break ground next year.

If Trump attempts to cut off funds to cities whose policies he does not like, it would set up a major battle in court, potentially altering the balance of power between the executive and legislative branches for good.

Trump had mixed success when he tried punishing sanctuary cities in his first term. A 2017 executive order barring such cities from receiving federal grants was struck down in federal court, although Trump ultimately prevailed with a narrow policy giving preferential treatment in awarding crime prevention grants to municipalities that cooperated with federal immigration authorities.

Trump wants the executive branch to have even more power, enabling him to satisfy his policy whims without needing to convince Congress to help him. He has floated the idea of challenging the 1974 Impoundment Control Act, which bars presidents from blocking federal funds appropriated by Congress. Proponents say the law was necessary to prevent executive overreach of the type President Richard Nixon engaged in.

“Not a very good act,” Trump said last year. “We will overturn it.”

His chosen “border czar,” Tom Homan, has also spoken in favor of punishing sanctuary cities, saying last week of the proposed deportation effort, “This is going to happen with or without you.”

Read more on the Trump team’s plans at The Washington Post.