Congress is on the verge of formally repudiating a president’s national emergency declaration, a historic first.

Last month, after losing a funding battle with Congress over his desire for a border wall, President Trump made his declaration, insisting that a “national security crisis on our southern border” precipitated his action. The Senate has mustered enough support to pass a joint resolution, introduced and approved in the House last month, calling for the immediate termination of a national emergency that represented an unprecedented end run around Congress’s power of the purse.

Mr. Trump has threatened to veto the resolution if it passes. Nonetheless, such a rejection from Congress would be healthy for the nation’s constitutional order. Among other things, it’d demonstrate that the separation of powers still matters and that some Republican lawmakers — after two years of looking the other way — are willing to stand up to a president with no respect for constitutional boundaries.

At the same time, the need for this legislative reprimand lays bare flaws in the National Emergencies Act, which Congress passed in 1976 to restrain executive power. Before the law, a president could declare a national emergency and unlock formidable powers under an array of statutes, giving him authority, according to a legislative analysis, “ to seize property and commodities , organize and control the means of production, call to active duty 2.5 million reservists, assign military forces abroad, seize and control all means of transportation and communication, restrict travel, and institute martial law, and, in many other ways, manage every aspect of the lives of all American citizens.”