In the nature of national emergencies, some definitional leeways are unavoidable. But Congress could readily specify certain conditions that must exist before the president can make such a declaration and thus arrogate to himself extraordinary powers — curtailing liberties, seizing property, spending funds appropriated for other purposes and suspending protective laws — that Congress would not otherwise be likely to grant him in advance, or perhaps ever. (Indeed, Congress has the power to override Mr. Trump's declaration and it should in this case — though it probably won’t.)

One can easily specify some of the factors in a national emergency condition that any responsible Congress would insist on before allowing such a declaration to take effect: magnitude (the feared incremental harms of the condition are very large, not just marginal); geographic scope (those harms should be nationwide even if the triggering condition is more localized); extraordinary (the condition should be rare, if not unprecedented); imminence (the anticipated harms should be so close in time that Congress cannot deliberate); and likely effectiveness (the president’s action should promise to substantially solve the problem).

Reasonable people may disagree, of course, about precisely how Congress should define and weigh these factors, how they should be applied to the facts on the ground and whether other factors should be considered. President Trump’s wall cannot survive such a rigorous analysis: Even assuming that recent illegal border crossings harm the nation, they are not greater (indeed, they are probably fewer) than in recent decades; Congress has been deliberating (in its fashion) for decades about immigration reform, including border walls; and many Americans believe that the wall would be ineffective and a bad idea. Congress simply disagrees with the president about which border security measures would do the job — an instance of decidedly normal politics.

But my larger claim — that Congress’s delegation to presidents of vast, essentially unconstrained power to declare national emergencies has been an irresponsible surrender of its constitutional responsibilities — does not depend on whether my assessment of this particular case is correct. Not since the New Deal has the Supreme Court struck down a statute for this kind of standardless delegation of legislative authority to administrative agencies, but the Roberts court — activist, conservative and suspicious of broad delegations of power to agencies — recently reached out to review a case, Gundy v. United States, that gives it an opportunity to revive the long-dormant and doubtful non-delegation doctrine.

Another way to make the National Emergencies Act more compatible with the rule of law is through a procedure that would broaden participation in all future decisions to declare national emergencies. Under one such reform, the president would have to consult with the leaders of both parties in Congress before issuing a declaration. Even if Congress could not override such a declaration, members would have to take a public position on the facts and reasons invoked as justification by the president. If time were of the essence (not so in this case; Congress has legislated about a wall for more than a decade), their responses would have to be expedited . With such a procedure in place, judicial review of the declaration could be narrow, if at all.