Here's what would happen if Trump declared a national emergency to build his border wall

• President Donald Trump appears increasingly likely to declare a national emergency to get funds to build a wall along the southern border.
• He has demanded $5.7 billion from Congress, which has refused to provide it, prompting a 21-day government shutdown.
• If he declared a national emergency, he would instantly be privy to a slew of special powers he could use to get the funds he needs.
• Such an action would almost certainly trigger challenges from Congress and the courts, but it's unclear how the battles will play out.

President Donald Trump said Thursday he "probably will" declare a national emergency to obtain the funds he needs for his long-promised border wall.

A partial government shutdown, sparked by a dispute between Trump and Congressional Democrats over the wall, reached its 21st day on Friday and is on the cusp of becoming the longest government shutdown in US history.

Trump has spent much of the last few weeks raging about a "crisis" he said has erupted at the US-Mexico border, propelled by of illegal immigration, drugs, and violent crime, which must be solved by constructing a physical barrier.

Critics, meanwhile, have argued that there is no crisis - or at least none that a wall can solve. Illegal border apprehensions are at a decades-long low, drugs mainly enter the US through legal ports of entry, and studies show that unauthorized immigrants commit fewer crimes than native-born Americans.

Read more: THE TRUTH ABOUT THE BORDER CRISIS: Experts say there is no security crisis, but there is a simple way to fix immigration - and it's not a wall

Nevertheless, Trump has been locked in a stalemate with Democrats over the wall that shows no signs of stopping, making it increasingly likely that Trump will declare a national emergency to bypass Congress for the wall funding.

Here's what you need to know.

What is a 'national emergency?'

A national emergency is something that the president declares to grant him special powers under the National Emergencies Act of 1976. Trump is hoping to use those special powers to allocate funding for the border wall.

There are 136 statutes governing which special powers the president can use, according to The Brennan Center for Justice, and it's unclear at this point which ones the Trump administration has in mind.

It's not uncommon for presidents to declare national emergencies. One of the most well-known examples is the national emergency that former President George W. Bush declared after 9/11, which is still in effect, and has been renewed by the sitting president each year.

Can Trump do this?

Experts are divided over whether it's legal for Trump to use a national emergency declaration for a wall.

But he faces relatively few restrictions on declaring a national emergency. According to The Brennan Center, 96 of the 136 statutory authorities available to presidents during national emergencies need only their signature, and just 13 require Congress to also declare an emergency.

Twelve of the authorities have a small restriction like requiring agency officials to certify that the measures are necessary, and the remaining 15 authorities require that the emergencies relate to particular subjects.

Yale law professor Bruce Ackerman wrote in a New York Times op-ed that there's no way Trump could use emergency powers to build the wall - at least if he opts to use funds from the military budget and use military personnel to build it.

Though there is some precedent for presidents' use of the military to enforce domestic law - primarily former President George W. Bush's authorization of the military to respond to Hurricane Katrina - that exception has since been repealed.

"Is President Trump aware of this express repudiation of the power which he is threatening to invoke?" Ackerman wrote.

Yet other experts said it might be easier for Trump to use national emergency powers than most think. Elizabeth Goitein of The Brennan Center for Justice wrote in The Atlantic that some of those 136 provisions available to Trump under the National Emergencies Act of 1976 appear "dangerously suited to a leader bent on amassing or retaining power."

"We are in uncharted political territory," Goitein wrote.

How would the Trump administration use these special powers?

News reports in recent days have indicated that the Trump administration is reviewing at least two options for how best to use emergency powers to secure the wall funding.

The White House has reportedly already asked the Army Corps of Engineers to review whether funds can be diverted from certain civil works projects in order to pay for the wall. NBC News reported that one of these projects could include reconstruction in Puerto Rico from the hurricane that struck the island in 2017.

The Trump administration is also examining whether the Department of Homeland Security can request the funds from the Pentagon - an idea that former Defense Secretary Jim Mattis backed before his resignation in December, according to The Wall Street Journal.

What could happen next?

If Trump does declare a national emergency, it's widely expected that he'll face major battles on two fronts: Congress and the courts.

Lawmakers from both parties have already expressed dismay over Trump's potential use of emergency powers. Some Republicans fear it sets a precedent that could later be used by a Democratic president to pursue liberal policies, while Democrats have called it a misuse of executive power.

One Democratic lawmaker, New York Rep. Grace Meng, even introduced legislation in the House on Friday to preemptively block Trump from potentially invoking a national emergency.

"We must send a clear message to the President that creating this type of manufactured emergency for the sole purpose of securing an unrealistic campaign promise is unacceptable," Meng said in a statement.

But a legislative challenge to Trump's national emergency powers could also hit roadblocks, thanks to a 1983 Supreme Court case that blocked Congress from using simple-majority votes to overrule a president's emergency declaration.

The National Emergencies Act of 1976 was then amended to require that both arms of Congress pass two-thirds majority votes to override a presidential veto.

Since some Republican lawmakers, like South Carolina Sen. Lindsey Graham, have already expressed support for Trump's use of a national emergency to build the wall, it may be unlikely that Congress puts up an effective fight.

But Trump is certain to also face multiple legal challenges, which could meet an unpredictable fate. The current Supreme Court has been reluctant to question some of Trump's executive powers - even deferring to him on the controversial travel ban he implemented.

Though plaintiffs in any lawsuits against Trump will likely try to question his administration's assertion that a national-security "crisis" is occurring along the southern border, the Supreme Court justices may well defer to his judgment on the issue, as they did with the travel ban.

"If any court would actually let itself review whether this is a national emergency, he would be in big trouble," Goitein told The Times. "I think it would be an abuse of power to declare an emergency where none exists. The problem is that Congress has enabled that abuse of power by putting virtually no limits on the president's ability to declare an emergency."

• Read more:
• Trump said he knows a border wall will work because wheels work, and the Secret Service uses 'really expensive' cars with wheels
• As the government shutdown over Trump's border wall rages, a journey along the entire 1,933-mile US-Mexico border shows the monumental task of securing it
• A lot of Republicans are worried about the dangerous precedent Trump could set with a national emergency declaration for the border wall
• The government shutdown is in a record-tying 21st day and the fight between Trump and Democrats is only getting uglier. Here's everything you missed.

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