By Priscilla Alvarez, CNN

(CNN) -- More than two weeks into the government shutdown, President Donald Trump is considering declaring a national emergency along the southern border as a way to pay for his border wall.

By doing so, Trump could unlock certain funds provided under statutes previously passed by Congress, such as those earmarked for natural disasters. By and large, the criteria to access those funds is specific. In the event of a national emergency, a president can circumvent Congress to use funds that haven't already been earmarked for emergency response. Anything beyond that would require Congress' approval.

In order to dip into agency funds and shift money either internally or to another agency, the administration would need congressional approval, which would be particularly challenging, given that Democrats control the House. Democratic Rep. Adam Schiff, the chairman of the House Intelligence Committee, pushed back on the idea of Trump using emergency powers on CNN's "State of the Union" Sunday.

"Look, 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 war on the border," Schiff said, referring to when the Supreme Court struck down President Harry Truman's attempt in 1952 to nationalize the steel industry. "So that's a non-starter."

If Trump does declare a national emergency, he would have access to a pool of money inside the Department of Defense. Under Title 10 of the US Code, Trump could use funds "that have not been obligated" within the Defense Department's budget.

The law states that in the event of a declaration of war or national emergency, "the Secretary of Defense, without regard to any other provision of law, may undertake military construction projects, and may authorize the Secretaries of the military departments to undertake military construction projects, not otherwise authorized by law that are necessary to support such use of the armed forces."

University of Texas School of Law professor and CNN contributor Stephen Vladeck noted that while this could provide some funds, it's still incremental, "This is not a blank check. It's a key to unlock different pools of money."

Defense Department officials told CNN that the Pentagon is planning a figure of about $2.5 billion in funds they believe they can tap to support construction of a border wall if Trump declares an emergency and orders the military to build a wall. Those funds fall under the "unobligated" pool.

A US official previously told CNN that Trump could use other funds but would have to cancel existing military construction projects, which might come with costly termination fees.

"If they canceled some activities in order to fundraise, essentially, for the border wall, they'd be incurring financial penalties in many cases," said Travis Sharp, a research fellow at the Center for Strategic and Budgetary Assessments. "It's not hard to imagine that if DOD would cancel one of the multibillion-dollar military construction projects outright, a fee would be easily in the millions."

The Defense Department currently has $13 billion in obligated funds from prior years that haven't yet been used. That's in addition to $10 billion in fiscal year 2019 funds that are set aside for other military projects.

In the event that Trump decides to pull from Defense Department funds, troops could be tasked with building parts of the wall, placing them and the department in a leading role where they've largely held a supporting role. They have assisted the Department of Homeland Security, which has long had jurisdiction over the border, - helping with construction projects and assisting Customs and Border Protection when necessary.

In an interview with ABC's "This Week" Sunday, Democratic Rep. Adam Smith, the chairman of the Armed Services Committee, acknowledged that the President can tap military money by declaring an emergency, but added that "primarily it's been done to build facilities in Afghanistan and Iraq."

During his presidency, Trump has repeatedly looked to the Defense Department for assistance along the border. In the run-up to the 2018 midterm elections, the administration deployed more than 5,000 troops to the border, and more recently, aviation support and assistance with installing concertina wire. Trump also previously flirted with the idea of the Defense Department fronting money for the wall.

Trump has argued that an increase in border apprehensions necessitates a wall and additional reinforcements to stem the flow of migrants. According to federal data, Customs and Border Protection apprehended nearly 400,000 people along the southwest border in fiscal year 2018, an increase from fiscal year 2017, but a decrease from fiscal year 2016 figures. Still, the administration has cited fiscal year 2018 figures as a point of concern.

Vladeck added that using Defense Department funding and personnel at the border also raises questions about what might suffer: "What are we taking DOD away from? If they're doing something new, it means they're not doing something old. Is there a broader concern of increasing DOD's operation footprint on US soil?"

In the past, unobligated funds have gone to things like construction projects that might arise throughout the year, like rebuilding a military base in the aftermath of a natural disaster, he added.

It's unclear if or how Trump would declare a national emergency. Asked about it on Monday during a briefing for White House reporters, Vice President Mike Pence said the White House counsel's office is currently examining the issue. A source familiar with the matter tells CNN that last year the White House Counsel's office under Don McGahn had examined whether the president could declare a national emergency over the border wall, but that no conclusion had been reached.

Legal experts noted that the details of any order would be key in terms of which statutes Trump plans on using to fund his border wall.

Individuals or groups "who can show they're directly injured by the action that comes as a result of a national emergency" could sue, according to Vladeck. The Democratic-controlled House could also file a lawsuit, arguing that the President is spending money that Congress hasn't authorized, but their claim would be limited.

Whatever the case, a national emergency likely wouldn't make the President's path toward a border wall any easier, and might instead incite more Democratic pushback and the possibility of legal challenges.

"In this case, I think the President would be wide open to a court challenge saying, where is the emergency?" Rep. Smith said on ABC. "You have to establish that in order to do this, but beyond that, this would be a terrible use of Department of Defense dollars."