The president is likely claiming authority to fund building the wall under the National Emergencies Act of 1976, but that law actually was meant to limit the ability of the president to claim powers by declaring a national emergency. One provision says that if there is a national emergency, funds in the Defense Department budget that are not “obligated” can be used for construction projects to support the armed forces. It says: “Secretary of Defense, without regard to any other provision of law, may undertake military construction projects, and may authorize 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.”
Source: latimes.com – Los Angeles Times