President Trump has threatened that if Congress does not appropriate money to build a wall along the border with Mexico, then he will build the wall through his national emergency powers. However, if Congress does not want President Trump to build the wall, he cannot build it.

The Appropriations Clause states that “[n]o Money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law.” The Supreme Court has interpreted the clause to mean that “the payment of money from the Treasury must be authorized by a statute.” The Supreme Court has even held that none of the president’s constitutional powers can override Congress’ power of the purse (“However large, therefore, may be the power of pardon possessed by the President, and however extended may be its application, there is this limit to it, as there is to all his powers — it cannot touch moneys in the Treasury of the United States, except expressly authorized by act of Congress.” emphasis added).

Nevertheless, existing law does potentially give President Trump the power to build a wall if he declares a national emergency. I will assume based on President Trump’s prior statements that he plans to use the national emergency authority in 10 U.S.C. 2808, which authorizes “[i]n the event of . . . the declaration by the President of a national emergency in accordance with the National Emergencies Act that requires use of the armed forces, the Secretary of Defense, without regard to any other provision of law, may undertake military construction projects . . . Such projects may be undertaken only within the total amount of funds that have been appropriated for military construction . . .”

For the purposes of this article, I will not delve into whether 10 U.S.C. 2808 actually gives President Trump the authority to build the wall because even if such exercise of emergency powers were to be upheld by a court, Congress can prevent President Trump from using the authority well before a court case could be decided.

Specifically, the the National Emergencies Act provides that Congress can terminate the president’s national emergency powers simply by passing a joint resolution. Further, the Act gives a joint resolution expedited, special privileges.

If President Trump were to declare a national emergency, the Democratic House of Representative would likely pass a joint resolution to terminate the national emergency. After the House passes its joint resolution, the Republican Senate would then have roughly 18 dates before it must vote on the joint resolution.

However, whether Senate Democrats could convince four Senate Republicans to support the joint resolution (and whether, as Professor Bruhl pointed out in the comments, Congress can override a veto) is largely an academic discussion as the House of Representatives has enough leverage to ensure that appropriations law prevents President Trump from building the wall.

In addition to the power to spend money, Congress has the power to prevent money from being spent. For instance, even though marijuana is illegal under federal law, appropriations laws have prohibited the Department of Justice from prosecuting medical marijuana, where it is legal under state law. In effect, appropriations law has made medical marijuana legal.

Likewise, House Democrats can ensure that the appropriations law that ends the shutdown completely encapsulates Congress and President Trump’s agreement on whether to fund a wall by (i) specifying that no money–or no additional money–from the law can be used to build a wall along the border with Mexico and (ii) amending the existing military construction appropriations (which is not shutdown) to say that no money from that appropriations or any previous appropriations can be used to build a wall along the border with Mexico, including under 10 U.S.C. 2808 authority.

While President Trump does have extensive national emergency powers, they are not absolute. As such, President Trump cannot use his emergency powers to build a wall along the border with Mexico without Congress’ consent.