Washington (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.

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