The firestorm over President Donald Trump's declaration of a national emergency to build a wall on the U.S.-Mexico border ignited with a fury Friday, as several groups vowed to sue the administration over what they've decried as an end-run around the Constitution.

Trump announced during a Friday press conference that he would declare a national emergency to begin construction of a wall. Much of the $8 billion dedicated for the project would come from diverting funds appropriated for other purposes, including the Defense Department's military construction projects.

“I didn't need to do this. But I'd rather do it much faster,” Trump said during a press conference Friday. Legal experts have said those remarks would be used in lawsuits to show that Trump's justifications for a national emergency declaration were nonexistent.

By Friday afternoon, several groups emerged claiming they would be a check on the Trump administration. None had yet filed lawsuits, but their arguments broadly were that the executive branch would write off Article I of the Constitution, that Trump has unlawfully redirected appropriated funds away from other federal programs, and that the declaration of an “emergency” was not justified.

“This is a patently illegal power grab that hurts American communities and flouts the checks and balances that are hallmarks of our democracy. We will be filing a lawsuit early next week,” Anthony Romero, executive director of the American Civil Liberties Union, said Friday.

The ACLU said it would argue Trump's use of emergency powers to override Congress' funding restrictions is “unprecedented” and that Trump lacked the statutory authority to use funds Congress dedicated for military construction projects to erect his border wall.

From the West Coast, California Gov. Gavin Newsom promised swift legal action, arguing Trump's plan would divert funds the state's law enforcement used to combat drug cartels “California will see you in court,” he said.

California Attorney General Xavier Becerra, a frequent legal foe of the Trump administration, also tweeted the day before that the state “will do what we must to hold him accountable. No one is above the law.”

Also vowing to join the fray are various states' attorneys general, including New York Attorney General Letitia James, who said Friday her office would use “every legal tool” at its disposal to fight Trump's emergency declaration.

Washington Attorney General Bob Ferguson teased similar legal action: “If Washington is harmed, my office will take appropriate steps to block this unlawful action,” he said Thursday in a press release.

Another group—Protect Democracy—said Thursday that it had prepared a lawsuit on behalf of El Paso County, Texas, and the Border Network for Human Rights, to challenge Trump. The group said it would work alongside the Niskanen Center.

Trump said his administration would likely lose any legal challenge to his declaration at the U.S. Court of Appeals for the Ninth Circuit. He said the case would then make its way to the Supreme Court, where “hopefully we'll get a fair shake and we'll win.”

Trump has been a frequent critic of the Ninth Circuit, which has frequently ruled against his policies. Trump did notch a victory at the Ninth Circuit over border wall construction earlier this week.

Looming over the president's declaration is the possibility of a court battle with the Democratic-controlled U.S. House of Representatives. Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer, in a joint statement, described Trump's move as an end-run around the Constitution and a violation of “Congress's exclusive power of the purse.”

“The Congress will defend our constitutional authorities in the Congress, in the Courts, and in the public, using every remedy available,” they said.

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