President Trump on Thursday sent strong signals that he is prepared to declare a national emergency in order to get his border wall built.

But it’s unclear whether he has the authority to use presidential emergency powers to obtain federal funding without congressional approval.

{mosads}Legal experts say the law could be interpreted in different ways depending on what steps Trump takes. But they agree that whatever move he makes is likely to set the stage for another high-profile courtroom battle, one that could very well come before the Supreme Court.

Here’s what you need to know.

What constitutes a national emergency?

The 1976 National Emergencies Act doesn’t clearly define what constitutes a national emergency.

“The statute is so broad it literally doesn’t have a standard,” said Bobby Chesney, a professor at the University of Texas School of Law.

That could give Trump the upper hand, at least initially.

Other experts note that courts often defer to the president when it comes to matters of national security.

For example, when the Supreme Court last year ruled on Trump’s travel ban, a majority of justices said the president had made a plausible argument that the policy was needed for national security reasons, even though he had called it a “Muslim ban.”

{mossecondads}Trump said this week that if he “can’t make a deal with people who are unreasonable” he will declare a national emergency, calling it his “absolute right.”

But he also said there is a “humanitarian and security crisis” at the southern border. In his prime-time address Tuesday from the Oval Office, Trump said the “southern border is a pipeline for vast quantities of illegal drugs,” including 90 percent of heroin that’s killing Americans.

That contrasts with a 2018 report from the Drug Enforcement Administration that said only “a small percentage of all heroin seized” by Customs and Border Protection “along the land border was between Ports of Entry.”

“How does a court react to something they know is false?” asked Harry Litman, a former U.S. attorney who teaches at the University of California, San Diego and UCLA School of Law.

Courts typically don’t want to be in the business of second guessing a president’s judgment, Litman said, while noting that this time could be different.

“The interesting legal question will be: Are they going to second guess Trump the man given his record of falsehoods or do they decline to second guess him in the interest of honest presidents to come, presidents of the future you will want to defer to?” he asked.

The outcome will set an important precedent for courts to follow if similar disputes arise in the future.

Who could challenge the declaration in court?

To have standing in court, a plaintiff would need to show they have been injured by Trump’s emergency declaration and that their injury could be rectified by a favorable decision from the court.

Chesney wrote in a Lawfare blog post this week that House Democrats could try to bring a case, but he noted that legislative standing is often difficult to establish.

“Other possible litigants with standing might include a landowner who faces eminent domain as a result of this,” Chesney wrote.

That could broaden the pool of potential plaintiffs given that the vast amount of land along the border is privately owned.

Another entity that could sue is a business that loses a government contract because funding is reallocated to the border wall, Chesney said.

Which other presidents have invoked emergency powers?

In 1952, President Truman famously declared a national emergency and ordered the secretary of Commerce to take possession of and operate most of the country’s steel mills after its workers went on a nationwide strike.

He argued that steel was needed for weapons and other war materials being used in the Korean War. That declaration was ultimately struck down by the Supreme Court a year later in a 6-3 ruling that said the president cannot take possession of private property to keep labor disputes from stopping steel production.

“This is a job for the nation’s lawmakers, not for its military authorities,” the court said.

President Reagan declared a national emergency to continue controls on certain exports after Congress failed to renew the pertinent law.

President George W. Bush notably declared a national emergency after the 9/11 terrorist attacks, giving him broad control over the military. Those powers were renewed in subsequent years during the Obama administration.

President Obama, meanwhile, issued a national emergency during an outbreak of swine flu in 2009 to allow hospitals and local governments to quickly set up sites for alternative treatment.

A Brennan Center for Justice report last month found there are more than 100 emergency powers a president can invoke when a national emergency is declared.

Where could the funding come from?

Democrats have refused to give Trump the $5.7 billion he’s demanding for his border wall, but declaring a national emergency could allow him to draw on funding that’s already been approved by Congress.

The National Emergencies Act requires the president to specify which provisions of the law he plans to invoke, and some experts say there are two statutes that might allow Trump to pull funds from military construction projects.

The first, 10 USC 2808, allows the secretary of Defense to use funds that have not been obligated for military construction projects so long as that money is used to support the armed forces.

But that raises of the question of whether the border wall is really in support of the armed services, according to Andrew Boyle, counsel in the Brennan Center’s Liberty and National Security Program.

He said a wall might not fall under the statute’s narrow definition of military construction, which it lists as anything necessary to produce a complete and usable facility.

Another provision, 33 USC 2293, allows the secretary of Defense to divert funds away from army civil works projects.

The White House said Thursday it is considering using $13.9 billion in funding Congress passed last year for various Army Corps of Engineers projects that have been allocated but not spent.