A police officer on the steps of the U.S. Supreme Court in Washington. (Salwan Georges/The Washington Post)

Richard H. Pildes is a professor of constitutional law at the New York University School of Law.

When the law under which President Trump has declared a national emergency was first enacted, in 1976, it was designed to require both Congress and the president to agree that a national emergency exists before the president could exercise emergency powers. Now that the Senate has joined the House in rejecting Trump’s emergency declaration, his declaration would be invalid — if not for a 1983 Supreme Court decision that destroyed the careful scheme Congress had designed for deciding whether emergency powers should be unleashed.

To understand how that Supreme Court decision dramatically affected the way the government works, consider what Congress had intended when it passed the National Emergencies Act. The law, created in the post-Vietnam and post-Watergate years, was one of many adopted in that era to constrain what had come to be called “the imperial presidency.” Congress decided that the awesome and dangerous power to declare emergencies required the agreement of both political branches of the government.

Recognizing that emergencies are unpredictable and often require immediate action, Congress established a formal process through which presidents could initially declare an emergency and trigger new powers for themselves, such as shifting the use of military funds. But at the same time Congress recognized the importance of reserving for itself a check on the president’s power.

Thus, Congress retained the power to end any presidential declaration of emergency. Congress could do that if the House and Senate passed a concurrent resolution to end the emergency. Crucially, a concurrent resolution, which in this context is also known as a legislative veto, is not a statute: The president has no say in such a resolution and cannot veto it. In essence, Congress gave the president the first word in declaring emergencies but retained the final word for itself — a power-sharing arrangement in the high-stakes matter of declaring national emergencies.

The Supreme Court’s 1983 decision in Immigration and Naturalization Service v. Chadha ended that arrangement. The court, in ruling on a challenge to the Immigration and Nationality Act, held that legislative vetoes are unconstitutional. The justices thus declared that nearly 200 statutes in which Congress, since the 1930s, had created a legislative veto were unconstitutionally structured. The National Emergencies Act was among them.

Rightly or wrongly (and over the vigorous dissent of Justice Byron White), the court concluded that Congress cannot act through a legislative veto but can act only by passing a new law. In the case of Trump’s declaration of a national emergency, that means Congress cannot simply disapprove, thus terminating the declaration, but instead must give Trump the chance to veto Congress’s disapproval, as he has promised to do. Instead of being able to end a presidential declaration of emergency with a majority vote in both the House and Senate, Congress must marshal an implausible two-thirds majority in each chamber to override the president’s veto.

Congress is often accused, justifiably, with fecklessly abdicating its institutional responsibilities by delegating vast, general powers to the president and the executive branch. Some critics of the National Emergencies Act have accused Congress of exactly that, in part because the act does not define what constitutes an emergency. But the criticisms are misplaced when it comes to the act in its original form.

Having resolved that deciding whether an emergency exists is ultimately a political judgment, Congress gave itself a crucial role in the process. The legislative veto sought to make Congress, not the courts, the appropriate institution to decide whether the president was right to declare an emergency. By their nature, emergencies are difficult to define; that challenge mattered less when Congress was empowered to react to specific circumstances and decide whether the president had appropriately declared an emergency.

Congress has now expressed its disapproval of Trump’s emergency declaration, but the Chadha decision means the vote has no direct legal effect. The inevitable presidential veto will almost certainly be sustained. Some senators are now trying to reclaim a more effective role for Congress by amending the law to terminate presidential emergency declarations automatically after 30 days unless Congress votes to approve them, but that is an issue for another day. In the meantime, those who oppose the president’s declaration have already filed suit. Traditionally, courts are uncomfortable when asked to second-guess presidential judgments in areas such as national security, foreign affairs and emergencies. But it was a decision by the highest court in the land that ensured the matter would be resolved in a courtroom and not by Congress.

Read more:

James Inhofe: Why I support the president’s national emergency

Danielle Allen: In voting against Trump’s emergency declaration, Congress can regain its power

Evan Thomas: The best way out of our national emergency mess? Let the system work.