New York City Mayor Bill de Blasio Bill de BlasioOVERNIGHT ENERGY: California seeks to sell only electric cars by 2035 | EPA threatens to close New York City office after Trump threats to 'anarchist' cities | House energy package sparks criticism from left and right EPA threatens to close New York City office after Trump threats to 'anarchist' cities New Year's Eve in Times Square to be largely virtual amid pandemic MORE (D) said Monday that he will fight President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE in court over Trump's plan to send undocumented immigrants to so-called sanctuary cities like New York.

"It’s illegal. It is just plain illegal. We will meet him in court. We will beat him in court," de Blasio said in an interview with NY1.

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"I remember vividly the day early in the Trump administration when he said he was going to cut our security funding because we did not ask for documentation status, because we would not cooperate with everything ICE was doing," he continued.

"We said we would go to court to stop him and we did. So this is just patently illegal. We'll stop it."

Trump argued Saturday that he has the legal right to send migrants to sanctuary cities — areas where local authorities refuse to help federal officials in enforcing immigration laws. The administration had previously denied such a plan was being floated.

"The USA has the absolute legal right to have apprehended illegal immigrants transferred to Sanctuary Cities," Trump tweeted. "We hereby demand that they be taken care of at the highest level, especially by the State of California, which is well known or its poor management & high taxes!"

He doubled down on the plan Monday, tweeting that "Illegal Immigrants who can no longer be legally held ... will be, subject to Homeland Security, given to Sanctuary Cities and States!"

Democratic Reps. Jerrold Nadler Jerrold (Jerry) Lewis NadlerDemocrats shoot down talk of expanding Supreme Court Schumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence MORE (N.Y.), Elijah Cummings Elijah Eugene CummingsBlack GOP candidate accuses Behar of wearing black face in heated interview Overnight Health Care: US won't join global coronavirus vaccine initiative | Federal panel lays out initial priorities for COVID-19 vaccine distribution | NIH panel: 'Insufficient data' to show treatment touted by Trump works House Oversight Democrats to subpoena AbbVie in drug pricing probe MORE (Md.) and Bennie Thompson Bennie Gordon ThompsonHouse panel pans ICE detention medical care, oversight Senate to hold nomination hearing for Wolf next week Hillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers MORE (Miss.), chairmen of the House Judiciary, Oversight and Reform, and Homeland Security committees, respectively, slammed the proposal on Monday as "bizarre and unlawful" in a request for documents regarding the plan.

Trump first said last week that he was "giving strong considerations" to the plan despite officials saying that the idea was never seriously considered.