In recent weeks, President Trump has sent signals that he is prepared to declare a national emergency in order to get his long-promised border wall built. On Jan. 4, Trump said, “We can call a national emergency because of the security of our country…I haven’t done it. I may do it…We can call a national emergency and build it very quickly.” On Jan. 6, he told reporters, “I may declare a national emergency dependent on what’s going to happen over the next few days.” But is such a move legal?

Under the U.S. Constitution, those powers it contains for dealing with urgent threats are assigned to Congress, not the president. For instance, Congress can suspend the writ of habeas corpus “when in Cases of Rebellion or Invasion the public Safety may require it” and “provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel invasions.” What’s more, Congress makes decisions about creating policy and spending taxpayer money under the Constitution. Article 1, Section 1 of the Constitution assigns the role of making laws to Congress, and Article 1, Section 9 stipulates that, “No Money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law.” The question, therefore, is whether there are existing statutory authorities that would allow the president to build a border wall.

In terms of construction authority, no statute allocates additional funding to the president during a national emergency. But legal scholars have cited two emergency powers that could allow the Trump administration to use Department of Defense funding: one statute makes available any unobligated funds originally set aside for military construction projects. This would only apply to national emergencies that require the use of armed forces, however. A second statute allows the president to divert funds from the army’s civil works program. While these statutes could give Trump the money to build the border wall, they do not give him authorization.

The 1976 National Emergencies Act provides affirmative authorization to the president to issue a national emergency. But it also outlines a framework for congressional involvement. For example, the Act requires that any declaration of national emergency be immediately “transmitted to the Congress and published in the Federal Register.” Moreover, the statute states that every six months “each House of Congress shall meet to consider a vote on a joint resolution to determine whether that emergency shall be terminated.” While these conditions were originally put in place to provide certain checks and balances on the emergency powers of the president, the Act has failed by any objective measure. According to a Brennan Center database, there have been at least 58 national emergency declarations since November 1979; 30 of which are currently in effect, including one related to the swine flu pandemic in 2009. During the 43 years the National Emergencies Act has been in place, Congress has failed to ever vote on whether to end them.

The Supreme Court has also shown deference to the president by upholding presidential emergency powers or by avoiding reviewing them. A notable example is Korematsu v. United States (1944), when the Supreme Court ruled, in a 6-3 decision, that Franklin D. Roosevelt’s internment of Japanese Americans was a “military decision” not based on race. Rulings such as Youngstown Sheet & Tube Company v. Sawyer (1952), in which the Court invalidated President Harry S. Truman’s effort to nationalize steel mills in the face of labor strikes, have been the exception.

And yet, commentators have argued that a national emergency declared by the Trump administration would almost certainly invite a court battle. “There’s no easy path for him here. It’s not a slam dunk—there would be a legal fight, to be sure,” Elizabeth Goitein, co-director of the Brennan Center’s Liberty and National Security Program, said in an interview with The Washington Post. In the event of a legal challenge, Trump would have to contend with a central legal question: is there a reasonable, good-faith basis for the declaration of a national emergency? Although the National Emergencies Act does not define “national emergency,” the Act indicates that a president may only declare an emergency that exists in fact. In other words, Trump would have to prove a crisis actually exists at the southern border—but empirical evidence suggests otherwise (e.g., the number of people crossing the border unlawfully is down, not up). To be sure, Trump’s public statements suggest he is looking toward a national emergency declaration as a political tool, to resolve a conflict with Congress. As the travel-ban litigation shows, the president’s public statements can invite examination of the motives behind executive action.

In conclusion, presidents have declared national emergencies since the Second World War. Today, it is unclear whether Trump will actually invoke a situation of national emergency. If he does, he will need to use every tool available to him as such a move could spark a flood of legal challenges.