As the partial government shutdown enters its third week, the White House is exploring other options to secure funding for the construction of a barrier on the U.S.-Mexico border, including using the president's emergency powers to circumvent Congress altogether.

"We can call a national emergency and build it very quickly," President Donald Trump said Friday. "But if we can do it through a negotiated process, we are giving that a shot."

The president repeated that assertion Sunday and acting White House Chief of Staff Mick Mulvaney said on CNN's "State of the Union" that Trump had directed "every single Cabinet secretary and the Office of Management and Budget to go out and find money that can be used legally to guard the southern border."

"Presidents have authority to defend the nation," Mulvaney said, adding that he was personally involved in the hunt to find funds that could be steered toward a border wall.

More:The government shutdown is tied for third longest on record – with no end in sight

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Can he do that?

So does the president have the authority to declare a national emergency and direct resources to the border?

"Trump can surely test whether he has the power to declare a national emergency for this purpose," said Kim Lane Scheppele, a professor at Princeton University's Center for Human Values. "The National Emergencies Act allows him to declare a state of emergency without approval from anyone else, but then he has to stay within congressionally delegated emergency powers after that."

The 1976 National Emergencies Act was passed in the post-Watergate, post-Vietnam era to create more structured procedures for the president's use of emergency powers.

Under that law, the president must cite the specific emergency powers he is activating under existing statutes. According to the Congressional Research Service, there are hundreds of "provisions of federal law delegating to the executive extraordinary authority in time of national emergency."

"Under the powers delegated by such statutes, the President may seize property, organize 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," says a 2007 CRS report.

Congress can terminate a declared emergency, but it requires a joint resolution – a high hurdle. House Democrats, now in power, would have to convince Senate Republicans, who still control their chamber, to join them in blocking Trump's move. Then they would have to get a signature from the president, the same person who declared the emergency in the first place, or override his veto.

"So the question on the border wall is: Is there any emergency power stashed somewhere in an already-existing law that could be stretched to include the movement of funds appropriated for one purpose to be used for another?" Scheppele told USA TODAY.

What kind of emergency would it be?

The president can declare an "immigration emergency" under the Immigration and Nationality Act.

An immigration emergency is defined as an "influx of aliens which either is of such magnitude or exhibits such other characteristics that effective administration of the immigration laws of the United States is beyond the existing capabilities" of immigration authorities "in the affected area or areas."

Other factors that determine if an immigration emergency exists include "the likelihood of continued growth in the magnitude of the influx," an "apparent connection between the influx and increases in criminal activity, high "demands on law enforcement agencies" and "other circumstances" that are defined by the attorney general.

Under the law, the attorney general or head of a local government must submit a request to the president describing the immigration emergency and what is needed to address it. The president then determines if an emergency exists and informs the House and Senate Judiciary committees.

An Immigration Emergency Fund exists for that purpose, but the law only approves $20 million annually for the fund, far short of the $5.6 billion Trump is seeking.

Another option that could potentially secure a larger chunk of funding for a border barrier would be under laws regarding military spending, which say the secretary of defense can undertake construction projects during a national emergency without authorization. The law says those projects must be "necessary to support such use of the armed forces."

The law also permits the Army’s civil works program to build structures necessary for the national defense after the president declares an emergency. The secretary of defense may also reprogram money that had been set aside for military construction projects.

A third statute allows for the construction of emergency projects if the secretary of defense determines it "is vital to the national security or to the protection of health, safety, or the quality of the environment," but that authority is capped at $50 million a year.

What do Democratic lawmakers think?

Rep. Adam Schiff, D-Calif., called Trump's invocation of his emergency powers "really threatening talk" that "he doesn't have the power to execute."

"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," Schiff told CNN.

But Truman did seize the steel mills in 1952, only to be blocked by the Supreme Court. Another Democrat, Rep. Adam Smith of Washington, said Trump did have the authority to declare the national emergency but, like Truman, he would face legal challenges.

"There is a provision in law that says the president can declare an emergency. It's been done a number of times," Smith told ABC's "This Week" host George Stephanopoulos. "In this case, I think the president would be wide open to a court challenge saying, where is the emergency? You have to establish that in order to do this."

Contributing: Christal Hayes