Updated 9:14 p.m. | President Donald Trump said Tuesday he would use the military to secure the U.S.-Mexico border, a complicated plan that could require him to declare a national emergency to avoid running afoul of a federal law that prohibits the military from acting as a police force.

“Until we can have a wall and proper security, we’re going to be guarding our border with the military. That’s a big step,” Trump told reporters during a White House appearance with leaders of Baltic countries.

Trump said he would be meeting with Defense Secretary James Mattis about the idea. Hours after his remarks, the White House said Trump was briefed on mobilizing the National Guard as well as other efforts to tighten border security.

In addition, the White House said Trump and senior officials including Mattis, Homeland Security Secretary Kirstjen Nielsen, Joint Chiefs of Staff Chairman Joseph F. Dunford Jr. and Attorney General Jeff Sessions “agreed on the need to pressure Congress to urgently pass legislation to close legal loopholes exploited by criminal trafficking, narco-terrorist and smuggling organizations.”

While neither the president nor the White House provided details on how such a plan would be executed, the idea encompasses several layers.

Federal law bars Trump from using the military as a police force, and while Trump could ask Congress for a waiver, that is unlikely because Senate Democrats, who oppose his border security agenda, can block passage of most legislation.

The National Guard was deployed to assist in border security operations by President George W. Bush in 2006 and President Barack Obama in 2010.

Watch: Trump’s Tweets Won’t Help Score a Border Security Deal with Congress

If Trump wants to use military funding to build his border wall, one option would be to repurpose funds granted by Congress to the Army Corps of Engineers. Normally, Congress would have to approve such a move, but Trump could invoke part of a 1986 law that would allow him to divert the funds by declaring a national emergency “that requires or may require use of the Armed Forces.”

The Army Corps received $2.1 billion for construction and $3.6 billion for operations and maintenance as part of the the fiscal 2018 omnibus spending law. It also has around $10 billion in unused funds, according to the Office of Management and Budget, as well as an additional $15.6 billion in supplemental disaster funding provided by Congress as part of the recent deal lifting budget caps for defense and nondefense. It’s unclear whether the disaster funding could be used for border security measures.

The administration is seeking between $18 billion and $25 billion for the border wall over 10 years, but has been stymied by Democratic lawmakers who have demanded concessions in the form of citizenship for “Dreamers” enrolled in the Deferred Action for Childhood Arrivals program.

The fiscal 2018 omnibus included $1.6 billion for border construction, but it could only be used to build steel fencing in use since the mid-2000s rather than the taller, concrete walls that Trump desires. Around 700 miles of barriers currently exist on the nearly 2,000-mile border, and the administration is eyeing more than 500 miles of additional construction.

Spokesmen for the House and Senate Appropriations committees said the White House has not reached out about repurposing the Army Corps’ funds.

Democratic lawmakers have already staked their opposition to Trump using funds appropriated to the Pentagon for border security. In a letter to Mattis on Monday, Sens. Richard J. Durbin of Illinois and Jack Reed of Rhode Island, the top Senate Democrats overseeing the Pentagon, wrote that Trump had no authority to use the funds for border wall construction.

“Based on a thorough review of appropriations law, … we conclude that the Department of Defense has no legal authority, with or without a reprogramming request, to use appropriated funds for the construction of a border wall,” Durbin and Reed wrote.

The Army Corps, which is funded through the Energy and Water appropriations bill, declined to answer specific questions about whether such a move is in the works. The Pentagon did not immediately return a request for comment.

Past operations

Military personnel, in the form of National Guard units, have twice been deployed in recent years as part of major operations to help secure the Southern border. From June 2006 to July 2008 during Operation Jump Start, guard units helped the Homeland Security Department conduct identification and engineering operations.

During Operation Phalanx, from July 2010 to September 2011, Guard personnel assisted DHS with identification, criminal analysis and command and control.

“We are always in support of that lead federal agency,” said Michael Kucharek, a spokesman for U.S. Northern Command, the military command whose area of responsibility includes the U.S.-Mexico border.

National Guard units generally operate either under state authority and funding, under a governor’s authority using federal funds or Defense Department funding and authority.

The Pentagon put its costs for Jump Start and Phalanx at $1.35 billion combined, according to a 2012 report by the Government Accountability Office. While National Guard members who were deployed were legally authorized to make arrests, the Defense secretary specifically blocked them from those activities to try to prevent the perception that the border was being militarized, the report said.

Customs and Border Protection officials reported that National Guard units helped apprehend 186,814 undocumented border crossers during Operation Jump Start, about 11.7 percent of total apprehensions that occurred while it was in effect. During Operation Phalanx, guardsmen helped apprehend 17,887 individuals, about 6 percent of the total.

Legal pitfalls

If Trump wants to use the military to secure the border, he’ll need to find a way around the law known as the Posse Comitatus Act, which limits a president’s ability to use military troops to enforce civil laws on U.S. soil. The 1878 law makes it a crime to use any part of the military to enforce a law unless Congress or the Constitution authorizes it, according to a Congressional Research Service report.

There are exceptions, such as legislation that allows the president to use military force to suppress insurrection or enforce federal authority, or laws that permit the Defense Department to provide federal and local police with information, equipment and personnel, the CRS states.

Congress enacted the law, first as an amendment to the Army appropriations bill, as a direct result of the Army’s involvement in Reconstruction in the South and President Ulysses S. Grant’s domestic mobilization of military forces during that time, according to two George Mason University law professors.

The most widely used exception is for National Guard forces that can be federalized, such as a 1957 directive to protect African-American students during the integration of schools or a deployment of troops to the Los Angeles riots in 1992, wrote the professors, Timothy MacArthur and Leigh Winstead, in an op-ed in The Hill about using troops for immigration enforcement.

The law’s limits on presidential authority may not be enough to prevent the militarization of immigration enforcement under a “national security” reason, the professors wrote.

“President George W. Bush nearly bypassed the PCA in such a manner after 9/11 when a Justice Department memorandum suggested that the President could unilaterally mobilize U.S. troops domestically to arrest a group suspected of plotting with al Qaeda because the operation was aimed at protecting national security rather than performing civil police enforcement,” MacArthur and Winstead wrote.