Donald Trump’s incoming “border tsar”, Tom Homan, has vowed to slash federal funding to states that do not cooperate with the administration’s promises of mass deportations of undocumented migrants.

Several states, including California, Illinois, New Mexico and Arizona have vowed to push back against Trump’s plans, and legal challenges are a near-certainty.

Trump has said he plans to use the use the US military to conduct the deportations, and Homan has said he “guarantees” that federal funding can be wielded as a tool against jurisdictions that stand in the way.

There are at least 13 million undocumented immigrants living in the US.

In an interview with Fox News, Homan – who was acting director of Immigrations and Customs Enforcement, or ICE, during Trump’s first term – said the Biden administration “turned this world upside down” with regards to immigration and the US-Mexico border.

“This is going to happen,” Homan said of the threat to cut federal funding to states that do not cooperate.

Across the US, Democratic-run cities and states have said they plan to resist the mass deportation plans.

Last week, Los Angeles’ city council passed a “sanctuary city” ordinance to bar using local resources to help federal immigration authorities.

Among the states whose governors have so far said they will not aid mass deportations are California, Arizona and New Mexico – three of four US states along the US-Mexico border.

• Migrants hope Trump mass deportations only ‘for criminals’

In Texas, on the other hand, authorities have said they are prepared to offer the president-elect and the incoming administration a 1,400-acre (567) hectare parcel of land for the “processing, detention and co-ordination of the largest violent criminals in the nation’s history”.

During his interview with Fox, Homan encouraged undocumented immigrants to “self-deport” to avoid the longer-term legal implications of being detained in the US and deported, such as being barred from re-entry for as many as 20 years.

“You should leave of your own will,” he said. “Even if you have a US citizen child, you can’t petition for it [if deported]. You can’t come as a visitor, or you can’t come on a student visa.”

“It’s best for these people to leave on their own so they don’t get an order of deportation that’s going to bar them from a legal entry in the near future,” he added.

Additionally, Homan said that deportees can choose to “leave the child here, or you can take them with you.”

The debate over mass deportations comes as unlawful crossings at the US-Mexico are hovering near record lows during the Biden administration.

Preliminary data obtained by CBS, the BBC’s US partner, suggests that US Border Patrol is on pace to record fewer than 50,000 apprehensions of migrants crossing the US-Mexico border in November.

In September, the number of detentions at the border stood at about 54,000 – far below the all-time high of 302,000 recorded in December 2023.

That would mark the lowest point since Joe Biden took office, as well as since the summer of 2020 when Covid-19 pandemic restrictions led to a sharp decline in the number of detentions.

While reduced detention numbers at the US-Mexico border could allow the Trump administration to focus resources on the mass deportation plan, it is still likely to run into significant logistics and financial difficulties.