Some of the president’s advisers have privately said that they share the longstanding opposition of conservatives to government intervention and oppose using the law, and the president again signaled his own ambivalence about it.

“When we need something, we’ll order something,” Mr. Trump said of the act. “As you know two days ago, I invoked the act. It is a big step. I am not sure if it is done before. When we need something, we’ll use it.”

Asked Friday night about specific ways the Defense Production Act has been used, the White House said in a statement that the president was “currently using it to drive the private sector’s response to this crisis,” and that he had invoked the act “to ensure that the necessary authorities will be available to prioritize production of items under government contracts and to allocate scarce items where they are needed most.”

In signing the executive order on Wednesday to put the act into effect, Mr. Trump said the purpose was to expedite distribution of “health and medical resources needed to respond to the spread of Covid-19, including personal protective equipment and ventilators,” and that Alex M. Azar II, the secretary of health and human services, could order production and distribution of supplies, if necessary.

But the president did not say if masks and ventilators in anything near the necessary quantity have actually been delivered to the workers who need them.

During Friday’s briefing, Mr. Trump grew increasingly confrontational with reporters as they pressed him on the details of the Defense Production Act, and he snapped at Peter Alexander, a reporter for NBC News, who asked him what he would say to Americans who were scared.

“I say that you’re a terrible reporter, that’s what I say,” Mr. Trump replied.

“It is a bad signal that you are putting out to the American people,” the president continued. “You want to get back to reporting instead of sensationalism. Let’s see if it works. I happen to feel good about it. Who knows. I have been right a lot. Let’s see what happens.”