What is a national emergency? Can Trump declare one for the wall?

US President Donald Trump has declared a national emergency in a bid to fund his promised wall at the US-Mexico border without congressional approval, an action Democrats vowed to challenge as a violation of the US Constitution.

The Republican president’s move to circumvent Congress represented a new approach to making good on a 2016 presidential campaign pledge to halt the flow of undocumented immigrants into the country.

Trump began weighing whether to declare a national emergency after Democrats refused to cave to his demand for $5.7bn in funding for the wall, prompting a 35-day government shutdown – the longest in US history. 

That shutdown ended last month after Trump, his fellow Republicans and Democrats came to an agreement to open the government for three weeks while talks continued on border security. 

Facing a Friday deadline, congressional leaders came to an agreement earlier this week aimed at keeping the government open. Although it includes $1.37bn in funding to help build physical barriers on the border, it does not include money for concrete walls, forcing Trump to look elsewhere for funding.   

Here’s a look at what a national emergency is, and why Democrats and many experts say Trump does not have the legal authority to declare one in this context. 

What is a national emergency? What will happen once it’s declared?

US federal law gives a number of powers to the president to use in response to a “crisis, exigency, or emergency circumstances threatening the nation”, a document prepared for Congress states. He can do so at will. 

Declaring a national emergency allows the president to meet the problems of “governing effectively” in times of crisis. They also allow the president to “seize property, organise and control the means of production, seize commodities, assign military forces abroad, institute martial law, seize and control all transportation and communication, regulate the operation of private enterprise, restrict travel, and, in a variety of ways, control the lives of United States citizens,” the document states.

Declaring a national emergency is a rare step. In this case, it would allow Trump to redirect federal funds from elsewhere to pay for his border wall. 

Jessica Levinson, political law professor at Loyola Law School, said although Trump had the power to declare an emergency at any moment, it would be a complicated process.

“He has to say here’s an emergency and here’s what I need,” she told Al Jazeera.

“I need more money, I need more people, etc,” she added. “But the question is then if he waited so long to do this, and the country went through a government shutdown over this, how is it an emergency now? Where is the urgency?”

It’s also unclear where Trump can get the funding because Congress would still need to earmark money for the wall. However, some legal experts have told local media that Trump may be able to turn to unallocated funds in the Department of Defense’s budget. 

A source told Reuters that the White House lawyers had vetted the figures and believed they would withstand a legal challenge.

Levinson said to expect a legal battle once Trump declares a national emergency. 

“At that point, people will sue and say this is not an emergency,” she said.

Fifteen Democrats in the Republican-controlled Senate introduced legislation prior to Trump’s declaration to prevent Trump from invoking emergency powers to transfer funds to his wall from accounts Congress has already committed to other projects.

New York state’s attorney general, Letitia James, said her office would also challenge Trump in court.

Under the Constitution, Congress holds the national purse strings and makes major decisions on spending taxpayer money.

Disputes over the constitutionality or legality of the exercise of emergency powers are judicially reviewable, the document prepared for Congress states.

What does Trump say?

Trump, who campaigned on a promise to build a wall on the US-Mexico border, had demanded Congress allocate $5.7bn in funding for the wall. 

But Democrats, who recently took control of the House of Representatives, refused, calling the wall ineffective, expensive and immoral. 

The legislation hammered out by Congress this week does not include money for a concrete wall, but it does allocate $1.37bn in funding for physical barriers. 

But Trump hopes by ordering the national emergency he can still follow through on his campaign promise. 

Trump estimated his emergency declaration could free up as much as $8bn to pay for part of the wall. Estimates of its total cost run as high as $23bn.

The president acknowledged that his order would face a lengthy court fight.

“I expect to be sued. I shouldn’t be sued … We’ll win in the Supreme Court,” Trump predicted as he announced the national emergency. 

On Monday, as congressional leaders announced a compromise that didn’t include wall funding, Trump declared in front of a rally of supporters that he was going to get the wall built one way or the other. 

“Just so you know – we’re building the wall anyway,” Trump said. 

The president maintains that the wall is necessary to stem irregular immigration and drug trafficking. 

But statistics show that irregular immigration has dropped to a 20-year low and that many drug shipments are likely smuggled through official ports of entry, leading critics to argue a wall is not needed. 

Confronted with those statistics at a Rose Garden event with reporters, Trump said they were “wrong”.

What do Democrats say?

Democrats have long resisted Trump’s call for a wall and said the president had fabricated a “crisis” at the border to justify an unconstitutional grab for funding.

Nancy Pelosi, the Democratic speaker of the House of Representatives, and top Senate Democrat Chuck Schumer swiftly responded to Trump’s declaration.

“The president’s actions clearly violate the Congress’s exclusive power of the purse, which our Founders enshrined in the Constitution,” they said in a statement.

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

On Thursday, Pelosi said there was no crisis at the border with Mexico that requires a national emergency order. 

“If the president can declare an emergency on something that he has created as an emergency – an illusion that he wants to convey – just think of what a president with different values can present to the American people,” Pelosi added, pointing to gun violence in the United States as a national emergency.

She said Democrats were prepared to respond appropriately to a declaration of national emergency. 

What do Americans say?

The majority of Americans continue to oppose expanding the border wall.

Of 1,505 adults surveyed last month in a Pew Research Center poll, 58 percent said they opposed substantial wall expansions while 40 percent said favoured wall expansions.

Partisan differences are widening as Republican support for the wall is at a record high at 82 percent, an increase of 10 percent from last year, while Democratic support has reached a new low at 6 percent, down from 13 percent last year.

Although opinion over wall expansions have not seen a substantial change from last year, they have never been more sharply divided along partisan lines.