There may be worse executive power abuses than President Donald Trump’s declaration of a national emergency to build a wall on the Mexican border that Congress has steadfastly refused to fund. But if there are, they don’t readily come to mind.

Trump’s national emergency declaration is worse than the 59 others declared by presidents since 1976 because it was employed to usurp the express power of the purse—the most important congressional check on executive abuses.

Only two of the prior national emergency declarations involved the expenditure of funds not appropriated by Congress: one by George H.W. Bush in 1990 during the run-up to the Persian Gulf War, and the other by George W. Bush in 2001 after the 9/11 attacks.

In both cases, the funds were transferred among military accounts in response to immediate (not protracted) developments for a military purpose. In neither case, had Congress repeatedly rejected the President’s proposed expenditure of funds, rather Congress acquiesced in the expenditures. In Trump’s case, he is expending funds for a purpose that Congress has consistently and steadfastly denied over long months. Trump is defying Congress, not working in collaboration with it, over a power Congress has a right to exercise.

Under the National Emergencies Act of 1976, Congress may pass a joint resolution to terminate the president’s declaration. If Trump persists, the United States Supreme Court can hold the executive usurpation of the congressional power of the purse unconstitutional. Further, Trump’s direction that funds be expended that have not been appropriated by Congress for that purpose would be criminal under the Anti-Deficiency Act and an impeachable high crime and misdemeanor.

The Anti-Deficiency Actcriminalizes the knowing authorization of a federal expenditure exceeding the amount it was appropriated for. In my opinion, a presidential crime that usurps a core congressional power constitutes an impeachable offense. Alexander Hamilton explained in Federalist 65:

The subjects of [impeachment] jurisdiction are those offenses which proceed from the misconduct of public men, or, in other words, from the abuse or violation of some public trust. They are of a nature which may with peculiar propriety be denominated POLITICAL, as they relate chiefly to injuries done immediately to the society itself.

Trump knows his national emergency demarche on the Constitution will shipwreck. He raced forward anyway to excite his base and to save face after coming out of the shutdown with less than he promised. Article I, section 9, clause 7 prohibits the president from spending money not appropriated by Congress: “No money shall be drawn from the Treasury, but in consequence of appropriations made by law.”

James Madison described the congressional power of the purse as the cornerstone against executive abuses. He elaborated in Federalist 58:

This power over the purse may, in fact, be regarded as the most complete and effectual weapon with which any constitution can arm the immediate representatives of the people, for obtaining a redress of every grievance, and for carrying into effect every just and salutary measure.

The power of the purse was used to terminate the Vietnam War after President Richard Nixon persisted in belligerency despite repeal of the Gulf of Tonkin Resolution (perhaps one of the great examples of this practice in modern times).

Accordingly, any presidential expenditure must be justified by a congressional appropriation. The president has no inherent Article II power to spend. Moreover, Congress is prohibited by the non-delegation doctrine from endowing the president with limitless discretion to declare a national emergency and spend money as he sees fit without any intelligible statutory standards. Thus, Congress could not, in the manner of Humpty Dumpty, make a national emergency mean whatever the president wants it to mean, neither more nor less.

Congress has never specifically defined a national emergency. But every definition in the English language requires circumstances necessitating an “immediate” response. Trump has conceded, “I didn’t need to do this, but I’d rather do it much faster.” In other words, there was no need for immediate action. Indeed, the president waited for months—including a 35-day government shutdown—before declaring a national emergency without any change in circumstances.

According to the Center for Migration Studies, most undocumented aliens in the United States have overstayed their visas. They have not illegally crossed the border and will be unaffected by a wall. According to the Trump administration’s own statistics, 80 to 90 percent of illegal drugs interdicted by law enforcement are at the ports of entry and unaffected by a border wall. Illegal immigration is at historic lows, and the incidence of crime among the undocumentedis less than among American citizens. These statistics help explain Trump’s inability to persuade Congress or a popular majority that a national emergency exists along the Mexican border.

Federal courts will be equally unpersuaded. As Justice Benjamin Cardozo observed, “The great tides and currents which engulf the rest of men, do not turn aside in their course and pass the judges by.” Thus did the Supreme Court repudiate Nixon’s bogus invocation of executive authority during the Vietnam War when he tried to suppress publication of the Pentagon Papers in New York Times v. United States (1971). Likewise did it reject President Harry Truman’s national emergency argument during the Korean War to seize steel mills in Youngstown Sheet & Tube v. Sawyer (1952).

The Democrat-controlled House of Representatives will likely sue to block the national emergency measure. The trailblazer case here is U.S. House of Representatives v. Burwell in the United States District Court for the District of Columbia. There, the Republican-controlled House was granted standing to sue over Affordable Care Act expenditures that had not been funded by Congress.

Only scrupulous adherence to constitutional processes will keep the law supreme in America. Justice Robert Jackson reminded us in Youngstown Sheet & Tube, “With all its defects, delays and inconveniences, men have discovered no technique for long preserving free government except that the Executive be under the law, and that the law be made by parliamentary deliberations.” It’s time for Trump to heed his words.

Bruce Fein was associate deputy attorney general and general counsel of the Federal Communications Commission under President Reagan and counsel to the Joint Congressional Committee on Covert Arms Sales to Iran. He is a partner in the law firm of Fein & DelValle PLLC.