WASHINGTON — President Donald Trump on Friday declared a national emergency at the U.S.-Mexico border, moving to secure more money for his long-promised wall by exercising a broad interpretation of his presidential powers that is certain to draw stiff legal challenges.

In his emergency proclamation, Trump painted a dark picture of the border as “a major entry point for criminals, gang members and illicit narcotics” and one that threatens “core national security interests.”

His declaration instantly transformed a contentious policy fight into a foundational dispute over the separation of powers enshrined in the Constitution, spurring talk of a congressional vote to block Trump and ensuring that the president and Democrats will continue fighting over the border wall in Congress, the courts and on the campaign trail.

It triggered outrage from Democrats and unease among some Republicans.

Trump signed the declaration to justify diverting billions of federal dollars from military construction and other purposes after Congress approved only a fraction of the money he had demanded. 

The standoff over border funding had led to the longest government shutdown in history. To avoid another shutdown, Trump reluctantly signed a funding bill Friday that included just $1.4 billion of the $5.7 billion he had demanded for the wall.

Trump announced the declaration in the Rose Garden, where he jousted with reporters and delivered a prediction about the fate of the order as it winds its way through the legal system.

“Sadly, we’ll be sued and sadly it will go through a process and happily we’ll win, I think,” said Trump.

Rep. Doug Collins, in a statement released shortly after the announcement, stood firm in the president’s camp:

“The crisis at our southern border demands immediate action, and I support President Trump’s commitment to securing our border and protecting our citizens. Declaring a national emergency is a serious act with deep implications, and it’s disappointing that partisan politics have brought us to this point. In acknowledging this, we also recognize the president possesses full legal authority to declare this emergency and he is not alone in his commitment to keeping Americans and migrants safe.”

Collins also called on Congress to continue working on immigration reform.

Kim Copeland, chairman of the Hall County Democratic Party, said he hopes the declaration is challenged in the court system.

“All of this is so President Trump can break his campaign promise, repeated numerous times, that U.S. taxpayers would never, ever have to pay a penny for his wall,” Copeland said in an email. “I believe this a constitutional crisis, and a great overstep of our executive functions.”

Within hours of Trump’s statement, the American Civil Liberties Union announced it would file suit challenging his emergency powers declaration.

“By the president’s very own admission in the Rose Garden, there is no national emergency. He just grew impatient and frustrated with Congress, and decided to move along his promise for a border wall ‘faster,’” said ACLU Executive Director Anthony Romero.  Some Democratic state attorneys general have also threatened to go to court over the decision.

The National Emergencies Act of 1976 allows the president to declare a national emergency without congressional approval. Thirty-one emergency declarations are currently in effect under the rule. Presidents have used the law to impose economic sanctions against terrorists after 9/11 and regulate foreign ships in U.S. waters, for example.

The top two Democrats in Congress said they’d use “every remedy available” to oppose what they cast as an unlawful measure.

“The President’s actions clearly violate the Congress’s exclusive power of the purse, which our Founders enshrined in the Constitution,” House Speaker Nancy Pelosi and Senate Minority Leader Charles Schumer said in a joint statement.

Trump defended his use of an emergency declaration, saying other presidents had done the same. Other presidents have used emergency powers, but not to pay for projects that Congress wouldn’t support.

And Trump himself sent mixed messages as to its necessity. He wrote in the official proclamation that “Because of the gravity of the current emergency situation, it is necessary for the Armed Forces to provide additional support to address the crisis.”

But he seemed to tip his hand at a political motive when he said during the news conference, “I didn’t need to do this, but I’d rather do it much faster,” an admission certain to be cited during legal challenges.

Republicans had opposed Trump declaring a national emergency, repeatedly warning that it would set a bad precedent and divide the party when Democrats put it up for a vote. While many in the GOP on Friday fell in line behind Trump’s decision, others remain opposed.

“I don’t believe a national emergency declaration is the solution,” Sen. Thom Tillis, R-N.C., said in a statement. “It wouldn’t provide enough funding to adequately secure our borders, it would likely get tied up in litigation, and most concerning is that it would create a new precedent that a left-wing president would undoubtedly utilize to implement their radical policy agenda while bypassing the authority of Congress.”

Congressional votes in coming weeks on a resolution blocking the emergency declaration are likely, but the timing is uncertain. Once a resolution is introduced, leaders by law cannot prevent votes on such a measure, which would need a simple majority to pass each chamber.

A resolution would all but certainly pass the Democratic-controlled House and may also pass the Republican-run Senate, if a few GOP senators break with Trump.  Congress seemed unlikely to muster the two-thirds majorities needed in each chamber to override a certain Trump veto. 

Staff writer Megan Reed contributed to this report.