But critics fear that weeks of dithering have already locked in deadly shortages at American hospitals.

Mr. Trump has “discretely, just in a discrete way, done something about ventilators. But there’s an overall appeal for him to do much, much more,” Speaker Nancy Pelosi said on Tuesday. “And this is nothing that can be handled piecemeal. It has to be across the board.”

Invoking the Defense Production Act is hardly a rare occurrence. As recently as last summer, the Department of Defense used it to obtain rare earth metals needed to build lasers, jet engines and armored vehicles.

The Defense Department estimates that it has used the law’s powers 300,000 times a year. The Department of Homeland Security — including its subsidiary, FEMA — placed more than 1,000 so-called rated orders in 2018, often for hurricane and other disaster response and recovery efforts, according to a report submitted to Congress in 2019 by a committee of federal agencies formed to plan for the effective use of the law.

The law, which was used frequently by previous administrations as well, does not permit the federal government to assert complete control over a company. The federal government can, however, use it to jump ahead of other clients or issue loans so a company can buy all of the supplies it needs to complete the government’s order by a specific date. A rarely used authority of the law also allows the administration to control the distribution of a company’s products and determine where such materials go.

The Pentagon has long been aggressive in its use of the law, inserting language from the wartime act into contracts to ensure delivery of products by a specific date.

For the coronavirus pandemic, Mr. Trump has elected to rely on the volunteerism of the private sector to obtain additional personal protective equipment, virus test kits and hospital equipment. He and his advisers have argued that using the act has been unnecessary, given the outpouring of support from large and small American companies — from Ford to MyPillow — that are retooling their factories to make masks, ventilators and gloves.