As Donald Trump promises the largest mass deportation of undocumented immigrants in US history if re-elected, his team is considering withholding federal police grants from local law enforcement agencies that refuse to participate in deportations, three sources said close to the Trump campaign for NBC. News.

The move could prompt legal challenges, as a similar policy did in the first Trump administration. The sources also cautioned that no plans are set in stone until Trump announces them himself.

The Trump campaign did not respond to requests for comment.

The three sources close to the campaign said the tactic would survive legal challenges and pressure blue states, counties and cities to participate in mass deportations.

Ron Vitiello, who served as acting director of Immigration and Customs Enforcement during Trump’s first term, recalled the deep frustration of administration officials when protracted court battles with Democratic-controlled states and cities blocked his plan Trump.

“The Department of Justice was going to withhold the grant money,” Vitiello told NBC News. “They didn’t enjoy, if you will, or fully participate in it.”

So-called sanctuary cities like Chicago and states like California currently refuse to alert ICE when they encounter or arrest undocumented immigrants. When undocumented immigrants finish serving their time in state prisons or local jails, they are generally released into the community rather than turned over to ICE.

Trump has vowed, if returned to office, to immediately begin rounding up undocumented immigrants across the country, starting with those who have criminal records or final deportation orders.

Nearly 1.5 million U.S. immigrants have final deportation orders, according to ICE officials. And more than 430,000 immigrants with criminal records are believed to be living in the country outside of ICE custody. It is not clear to what extent the two groups overlap.

During the first Trump administration, more than 400,000 immigrants with criminal records lived in the US outside of ICE detention, about the same number as under the Biden-Harris administration.

If federal agents were to arrest and deport up to 1.9 million such immigrants it would cost nearly 10 times ICE’s current annual budget, according to the American Immigration Council, a pro-immigration research and advocacy organization.

To pressure local law enforcement agencies to cooperate, the Trump team would cut off their access to Justice Department grants, the three sources said. The Justice Department describes the grants, known as the Byrne JAG grant program, as “the primary source of federal justice funding for state and local jurisdictions.”

In the past, the Department of Justice has awarded more than $250 million in funding per year to state and local law enforcement agencies through the Byrne JAG grant program. The process is competitive, and law enforcement organizations have used the funding to cover the costs of policing drug rehabilitation centers and corrections programs.

During Trump’s first term, several states, including New York, sued the administration after it cut off Byrne JAG grants to sanctuary cities. After an appeals court sided with the Trump administration, Democratic-controlled states appealed to the Supreme Court. And that legal maneuver essentially ran out the clock on the first Trump administration.

Before the Supreme Court ruled, Joe Biden won the 2020 election, took office and revived the sanctuary city grant program. Trump campaign officials say they are confident they can quickly get legal challenges to the Supreme Court and that the high court’s conservative supermajority will back them.

Vitiello said a second Trump administration would also provide additional funding to state and local police departments that participate in mass deportations.

“We could see, under the administration, a boost,” Vitiello said. “Or a way to provide leverage for those who don’t want to comply.”