What is particularly disturbing about this procedure is that the National Emergencies Act effectually empowers presidents to retain emergency authority until they decide to give it up. An emergency declaration can be renewed by the president and terminated only by a joint resolution of Congress that requires the president’s signature. The expectation that presidents will voluntarily renounce emergency authority runs contrary to all assumptions the constitutional order makes about the seductions of power, which, as Federalist 48 noted, is “of an encroaching nature.”

It is especially disturbing that Mr. Trump was goaded into invoking the Korean War-era Defense Production Act, which enables presidents to direct industrial production toward war needs when national security or natural disaster requires it. The fact that the act has been amended to apply to domestic emergencies underscores the tendency of emergency powers to expand. What about Mr. Trump’s erratic response to the coronavirus pandemic thus far — from his early flattering of China to his repetitive efforts to minimize the crisis — inspires confidence in his ability to command the industrial capacity of the nation?

Even if one granted the defensible premise that the national government needs to do more to force industry to produce emergency medical supplies, equating domestic crisis with foreign war is dangerous. War powers are notoriously difficult to contain once unleashed. They tend, instead, to metastasize, as in a recent Justice Department request to allow indefinite detention without trial during emergencies. More broadly, crisis powers tend to remain in presidential hands once the immediate danger passes, especially when wars do not have clear beginnings and ends.

Witness the fact that the post-9/11 Authorization for Use of Military Force remains in effect nearly 19 years later. Americans born after it was enacted will be eligible to vote in the next election. They are also serving in the seemingly endless conflicts that have ensued. In the case of 9/11, the preoccupation was with emergency action at all costs. We are still grappling with the constitutional fallout a generation later.

The coronavirus crisis could prove even more insidious. Mr. Trump’s use of the National Emergencies Act to help fund his border wall shows how tempting that kind of power can be. If war statutes are converted to domestic use, Democrats might consider what will happen if Mr. Trump — who has already likened illegal immigrants to invaders — declares a metaphorical war on them.

Similarly, Republicans should survey the powers being conferred on Mr. Trump and ask whether they would be comfortable with Joseph R. Biden, the presumptive Democratic nominee, for president, invoking national security powers for urgent “wars” that address his priorities, like climate change or gun control. It may be a far leap from here to there. It is also now a shorter one.

None of these is equivalent to the genuine and immediate crisis the coronavirus presents. But all of them serve as warnings that the powers will not easily go away when this crisis ends. Far from it: We risk becoming inured to them and legitimating their future use.