Mexico won't pay for it. It won't necessarily be made of concrete. And Donald Trump now says he won't even need congressional approval to get his $5.6 billion to fund construction of a southern border wall — not if he declares a "national emergency" to do it.

The U.S. president may do so at his own discretion, though experts on executive authority say it's legally dodgy to invoke such presidential powers using a non-wartime rationale.

"I may declare a national emergency, dependent on what happens over the next few days," Trump told reporters on the weekend.

Trump's threat to take radical action to fund his wall comes amid an impasse that has plunged the federal government into an 18-day shutdown. He plans to deliver a prime-time Oval Office address on Tuesday night.

"Presumably, he'll be laying out a case for a national emergency. It seems like he'll lay out a more extensive case," said Jon Michaels, an expert on presidential powers with the University of California, Los Angeles.

If Trump does invoke his power to take control of the purse strings from legislators — despite the constitutional mandate that no treasury funds shall be spent unless first appropriated by both houses of Congress — he should expect court challenges.

Here's what to know about presidential use of national emergencies.

What is a national emergency?

Executive powers, as outlined in Article II of the U.S. Constitution, give the president broad authority to protect the public and preserve national security in times of crisis.

Eight prototypes for the border wall are shown in San Diego in late 2017, when they were awaiting tests to determine how easily they could be tunnelled under, climbed over or chiselled through. (Paul Hunter/CBC) (Paul Hunter/CBC)

The National Emergencies Act of 1976 was enacted during the post-Watergate era to require presidents to mount a legal case for exercising such powers.

According to a 2007 Congressional Research Service report, presidential powers are expanded "in the event that the nation is threatened by crisis, exigency or emergency circumstances," such as natural disasters, wars or "near-war situations."

Does the need for a border wall rise to that level?

Many legal scholars are skeptical. They argue the authority is reserved for situations of unforeseen emergency.

Trump often calls unauthorized immigration at the southern border a "crisis." But James Thurber, founder and former director of the Center for Congressional and Presidential Studies at American University, says the issues haven't presented a sudden, overpowering period of upheaval that can only be resolved through a physical wall or steel slats, rather than by, say, more border guards or new technologies.

Thurber leans on a 1934 Supreme Court definition of an emergency as a disaster or "some unusual occurrence not reasonably subject to anticipation."

The situation at the border is not "something that just occurred," he says. "It's not overwhelming the entire United States. It's not threatening life and limb … and the data don't show it's getting dramatically worse."

Trump would likely have to prove there's a clear and present danger at the nation's doorstep to justify invoking emergency measures.

What are some historical examples?

From the founding of the U.S. to the present day, there have been hundreds of examples of presidents using emergency powers.

"They're almost all dealing with national security-related threats by foreigners, or foreign policy, or trade policy, or temporary treaties," Thurber said. "But nothing like this [demand] for a wall."

Perhaps the earliest known example of emergency action was with George Washington in the 1790s, during the Whiskey Rebellion, when distillers in western Pennsylvania began to forcefully oppose federal excise taxes.

U.S. President Donald Trump is considering declaring a national emergency in order to get his border wall funded. Disagreements about how to fund the wall have already led to a partial government shutdown that Trump is in no rush to end. 3:00

Washington dispatched a militia to shut down the rebellion.

And Franklin Roosevelt controversially signed Executive Order 9066 in 1942, three months after the Japanese bombed Pearl Harbor, to authorize the creation of military zones for the relocation and incarceration of some 110,000 Japanese-Americans in internment camps. (That act is widely viewed as unconstitutional today.)

More recently, it was a declared state of emergency by George W. Bush after the Sept. 11 attacks that precipitated the establishment of a detention centre at Guantanamo Bay in Cuba, and also led to warrantless wiretapping and surveillance.

Who can restrict the president's emergency powers?

There's little to stop him from at least wielding his authority in the short term.

"If the president were to decide to exercise emergency powers, it's difficult to see how that could be halted immediately," said Meena Bose, executive dean of public policy at Hofstra University.

He can still be reined in through judicial or legislative recourse, but both processes take time. A legislative solution appears highly unlikely, given the Republican majority in the Senate. (More on that later.)

"Even if a case is moved forward expeditiously, the president still has power to take action," Bose said.

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Democrats have promised to challenge Trump in court if he declares a national emergency.

Court cases can be elevated quickly. During the Florida presidential recount debacle in 2000, for example, the Supreme Court settled the case of Bush v. Gore by Dec. 12, five weeks after the election.

Still, even Democrats reluctantly agree Trump probably has the power to declare a national emergency over border security. Adam Smith, the incoming Democratic chairman of the House Armed Services committee, acknowledged as much when asked about the issue on ABC News.

"Unfortunately the short answer is yes," he said, though he warned Trump "would be wide open to a court challenge."

How broad are emergency powers?

They range "from the minute and nitpicky to the jaw-dropping," said Liza Goitein, with the Brennan Center for Justice at New York University.

The Brennan Center tallied up 123 statutory provisions that the president can access upon declaring a national emergency. (An additional 13 emergency powers open up to him if Congress declares a national emergency.)

Human rights activist Fernando Garcia, from the Border Network for Human Rights, says any wall that would be built by Trump is symbol of racism and hate in America. (Jason Burles/CBC)

Goitein gave the example of a statute allowing the president to shut down "wire communication," which could be interpreted as the ability to control American web traffic. Another provision allows U.S. Coast Guard officers to serve as notaries public during an emergency.

With respect to the border wall, Goitein believes Trump's legal team will be examining two emergency provisions in particular: a law that would allow his secretary of defence to reprogram money around the Department of Defence and another that would authorize military construction projects not authorized by Congress.

So can Trump get his wall funding without Congress?

Technically, there is a path.

The Department of Defence has "un-obligated" funds — money set aside for unspecified military construction — that might be dipped into for building a wall, said Mark Tushnet, a constitutional law professor at Harvard Law School.

"That's a pot of money the Department of Defence can draw on to do things that haven't been specified, of which the wall [construction] would be one example."

Vice President Mike Pence, center, looks on as House Minority Leader Rep. Nancy Pelosi, D-Calif., and President Donald Trump argue during a meeting in the Oval Office of the White House, Tuesday, Dec. 11, 2018, in Washington. (AP Photo/Evan Vucci) (Evan Vucci/Associated Press)

While that military construction is likely intended to mean temporary housing for soldiers, Tushnet said the president can declare a national emergency with respect to the border, then follow the procedural steps of defining what the border emergency is, as required under the National Emergency Act.

"When he does that, he can then use the 'un-obligated funds' in the DoD budget for military construction. And his lawyers would argue that given the nature of the emergency, a wall counts as military construction."

Will it be an easy path?

Not with the court challenges expected.

During the Korean War, the Supreme Court ruled against President Harry Truman when he tried to seize control of the steel mills amid an imminent strike. And that was during wartime, Democratic Rep. Adam Schiff noted on CNN.

"If Harry Truman couldn't nationalize the steel industry during wartime, this president doesn't have the power to declare an emergency and build a multibillion-dollar wall on the border," he said.

What's the fastest way to resolve this if Trump declares an emergency?

If Trump invokes emergency powers, the swiftest resolution is probably a legislative solution.

To get there, Congress would have to pass a funding bill that doesn't include funding for the border wall and also overrides the president's veto with two-thirds majorities in both chambers.

"It's a high bar," Bose said. Historically, Congress has overridden less than 10 per cent of all presidential vetoes.