Donald Trump has warned that he could declare a “national emergency” on the US-Mexico border, where a row over the funding for his proposed wall has triggered a US government shutdown.
Tapping into emergency presidential powers could enable Mr Trump to bypass Congress and access the money and resources needed to complete the project.
But what are these emergency powers, and is using them that simple?
What exactly is a state of emergency?
A state of emergency is declared in times of crisis. In this case, Mr Trump says the crisis is being caused by migrants arriving on the US-Mexico border.
According to Elizabeth Goitein, co-director of the Brennan Center’s Liberty and National Security Program, declaring a national emergency gives the president “access to special powers that are contained in more than 100 other laws”.
Those powers effectively allow the president to bypass the usual political process.
“Obviously, the intent is to provide for badly needed flexibility when there are urgent crisis which Congress does not have time to address,” she explained.
But is the southern border really an emergency?
While both sides agree it is a problem, whether it constitutes an emergency is a point of debate.
On the one hand, more than 2,000 people were turned away or arrested at the border each day during November alone. Supporters say this equals an emergency. Others argue the figure is far lower than a decade ago, and many of the thousands of people who travelled north from countries like Honduras are presenting themselves as asylum seekers, looking to enter the country legally.
Ms Goitein, an expert on the president’s emergency powers, does not believe it reaches the bar.
“It needs to be something fast-moving, totally unforeseen,” she argued. “It is meant to be a stopgap measure.
“This is not such a thing and it would be a tremendous abuse of power to invoke just to short circuit the political process.”
She does, however, point to two sections of the law which Mr Trump – who aims to use the power to free up the money needed for the wall – could possibly use.
One will allow the redirection of funds for military projects already approved by Congress. The other would require the administration to prove the wall amounts to a military construction.
Neither, Ms Goitein notes, are a “slam dunk” for the president, who must cite one of the laws as the legal basis for his declaration.
So, Trump could build an emergency wall?
It is technically possible. However, Mr Trump’s opponents and civil liberties groups are unlikely to submit to it without a fight.
First of all, Congress could vote against the move. How likely is that? It is unclear. But Chris Stirewalt, the political editor of Fox News, the US channel which has thrown itself behind the border wall, said he could not see Congress allowing it to pass when asked about the situation on Monday.
If it does get through, there is still the option of blocking the move through the courts – which is what happened when President Harry Truman tried to nationalise the steel industry during the Korean War, setting a legal precedent.
As Adam Schiff, the House intelligence committee chairman, told CNN: “If Harry Truman couldn’t nationalise 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.”
How frequently do presidents use emergency powers?
Surprisingly often. For example, Mr Trump’s predecessor, Barack Obama, used it 13 times, while his predecessor George W Bush used it 12 times. If Mr Trump goes ahead with declaring a national emergency, it would be the fourth of his administration.
Some uses were for issues like the H1N1 influenza epidemic, and have since ended. Others are more general and continue to this day, like blocking the property of people “engaging in significant malicious cyber-enabled activities”.
Perhaps the most well-known occasion a president used his powers was in the case of Franklin D Roosevelt, who used them to order the internment of more than 110,000 Japanese-Americans in the months after the 1941 attack on Pearl Harbour.
George W Bush, meanwhile, used emergency powers to sign off wiretapping and controversial interrogation methods decried as torture following the 9/11 attacks, Ms Goitein said.
So what are the long-term implications of Trump building a wall?
Well, for a start it could mean the US has to find billions of dollars to pay for the wall in the not-too-distant future. Despite Mr Trump’s earlier assurances, it doesn’t seem like Mexico is about to offer up the cash.
But it could also serve as a wake up call to Congress about the level of power granted to presidents through the 1976 National Emergencies Act.
“All this shows is why the legislation cannot place such unfettered trust in a president,” Ms Goitein said.