President Donald Trump on Friday declared a national emergency to unilaterally build barriers along significant portions of the United States-Mexico border.

The Justice Department has reportedly warned the White House that the courts are almost certain to at least temporarily block the emergency declaration.

The border-security compromise package Congress passed on Thursday provides $1.4 billion for barrier construction.

WASHINGTON — President Donald Trump on Friday declared a national emergency to address issues with border security and unilaterally build barriers along significant portions of the United States' border with Mexico.

The action, which ventures into new territory to circumvent Congress and redirect military funding for other purposes, has drawn condemnation and confusion from lawmakers in both political parties.

Trump made the announcement in the White House's Rose Garden, saying: "I'm going to be signing a national emergency. It's been signed many times before."

"They say walls don't work," Trump said. "Walls work 100% of the time."

Trump also downplayed the accusation that he could not cut a deal with Congress, saying he was able to get $1.4 billion for barrier construction in the spending bill it passed Thursday but was still not satisfied.

Trump also acknowledged he is likely to face considerable legal challenges for taking executive action.

"I expect to be sued," he said. "I shouldn't be sued."

In a joint statement responding to the emergency declaration, House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer described it as unlawful and said it "does great violence to our Constitution and makes America less safe, stealing from urgently needed defense funds for the security of our military and our nation."

"The President's actions clearly violate the Congress's exclusive power of the purse, which our Founders enshrined in the Constitution," they said. "The Congress will defend our constitutional authorities in the Congress, in the Courts, and in the public, using every remedy available.

They added: "The President is not above the law. The Congress cannot let the President shred the Constitution."

Read more: Trump praises several conservative media pundits during emergency declaration speech in Rose Garden

In a conference call with reporters before the announcement, senior administration officials detailed the plan, saying that in addition to the $1.4 billion for barrier construction appropriated by Congress, about $6 billion would be taken using Title 10 of the US code and $600 million would be tapped from the Treasury's forfeiture fund.

A senior administration official said the construction would follow the same design as the one appropriated by Congress, using a steel bollard wall instead of concrete or other prototype walls.

"There's not a fight over what's going to be built," an official said.

The acting White House chief of staff, Mick Mulvaney, also downplayed the assertion that what Trump is doing is unprecedented.

"There's been some concern in the media about whether or not this creates a dangerous precedent," Mulvaney told reporters. "It actually creates zero precedent. This is authority given to the president in the law already. It's not as if he just didn't get what he wanted so he's waving a magic wand and taking a bunch of money.

"I saw Nancy Pelosi said yesterday this sets a precedent for the Democrats to declare a gun emergency the next time they're in the Oval Office. That's completely false," he added. "If Democrats could've figured out a way to do it, they would have done that already."

Trump's decision confounded Republicans worried about an overreaching executive

Rep. Mac Thornberry of Texas, the ranking Republican on the House Armed Services Committee, in a Thursday statement urged Trump "not to divert significant Department of Defense funding for border security."

"Doing so would have detrimental consequences for our troops as military infrastructure was one of the accounts most deprived during the Obama-era defense cuts," Thornberry said. "And it would undercut one of the most significant accomplishments of the last two years — beginning to repair and rebuild our military. I hope that the President will pursue other options."

On the other hand, Republicans loyal to the president, like House Minority Leader Kevin McCarthy, praised Trump for taking swift action.

"We face a humanitarian and national security crisis at the border that must be addressed and the President's declaration is merely a statement of fact," he said in a statement. "With the declaration and other legal authorities, the President has access to important tools to take the steps necessary to secure the border."

Legal challenges are almost certain

Long before Trump made the decision, and as he was hanging it over Congress' head during the government shutdown, Republicans questioned the legality of an emergency declaration, including supporters of the move, like Sen. Lindsey Graham of South Carolina.

Democrats have vowed to fight it and said they are confident the White House will be rebuked by the courts.

"A declaration of national emergency would be absolutely repugnant and abhorrent to our constitutional principles," Democratic Sen. Richard Blumenthal of Connecticut told INSIDER in January. "It would lack any basis in fact or the law and would be struck down by the courts."

The Department of Justice has warned the White House that the courts are likely to block the emergency declaration at least temporarily, ABC News reported on Thursday.