President Trump was eager to make another headline-grabbing pronouncement Thursday in his administration’s frantic fight against the coronavirus outbreak when he asserted that an ­anti-malaria drug called chloroquine, which had shown potential benefits to treat the illness in overseas trials, would be ready for prescription use in the United States “almost immediately.”

Moments later, FDA Commissioner Stephen Hahn followed him in the White House briefing room and gently put the brakes on the president’s zeal. In fact, Hahn explained, his regulatory agency would need to conduct a “large, pragmatic clinical trial” to ensure the treatment would be both safe and effective for patients.

Doing otherwise, he added, “may do more harm than good.”

The exchange was the latest example of a dynamic that has played out this week as Trump has sought to demonstrate that, after initially minimizing the threat of the pandemic, he has pivoted to a wartime presidency footing and sought to marshal the full power of the federal government to dampen the rapid spread of a lethal illness.

Day after day, many of the president’s boldest pronouncements at the White House have failed to materialize as quickly as he has promised or turned out to be more complicated than he has suggested. The upshot, Trump’s critics said, is that he has contributed to widespread confusion and uncertainty among the public he is attempting to reassure.

“The president’s press conferences seem to be this substitute for what he would typically do at political campaign rallies, in which he floats a bunch of things out there to see what sticks and then much of that information is not accurate or entirely based in fact,” said Graham Brookie, a former National Security Council staffer in the Obama administration who now oversees an Atlantic Council program on governance, technology, security and social media.

“That, in and of itself, makes mounting a public health response much more difficult,” he added.

Trump said this week that the military had dispatched a pair of floating hospitals on naval vessels to help overwhelmed jurisdictions, including New York, which has the highest number of confirmed virus cases in the nation. But the ship that is being routed toward the Big Apple, the USNS Comfort, remains at least two weeks from being ready for service, Pentagon chief Mark T. Esper said.

The president directed Americans to consult a new Google website aimed at screening patients who might need coronavirus testing, but it turned out to be a pilot project limited to the San Francisco Bay area. He announced he would invoke the Defense Production Act that would empower the federal government to force private industry to produce critical medical supplies such as respiratory masks and gloves — but then acknowledged he had not yet chosen to use the powers, saying it remains up to governors and local officials to procure such items.

The president has said repeatedly that Americans who need coronavirus tests can get them and vowed that his administration is rapidly scaling up production of testing kits. Yet the nation remains well behind China, South Korea, Italy and other countries in testing.

Asked Thursday about the gap between his rhetoric on testing and the pleas from hospitals across the nation that testing is severely lagging, Trump failed to come up with a cogent explanation.

“Well, I can’t — I cannot explain the gap,” he replied. “I’m hearing very good things on the ground and we are — look, they had a ramp up.”

He suggested the nation’s health-care system was “obsolete” and not prepared to deal with a pandemic of this magnitude. “Nobody’s ever seen anything like this before,” Trump said, even though his administration had conducted an assessment exercise last year about dealing with a potential pandemic originating in China, according to the New York Times.

That exercise produced a draft report that concluded the administration was underfunded and unprepared for such a scenario.

“While we’re facing a pandemic, we’re also facing what the WHO refers to as an ‘infodemic’ — an overabundance of information. Some of it is accurate, and some is not,” said Brookie, the Atlantic Council analyst, referring to the World Health Organization. He said the president would better inform the public by emphasizing the government website, coronavirus.gov, that is dedicated to the most up-to-date information. Vice President Pence has begun touting the site, but Trump has not.

As the rapid spread of the virus has forced the cancellation of a wide swath of public life and sent financial markets tumbling, Trump has grown increasingly worried about the impact on his reelection campaign and has moved to exert full control of the administration’s public messaging, appearing at the White House’s daily coronavirus briefing.

In doing so, he has at times appeared to push aside key medical figures, including Robert Redfield, the director of the Centers for Disease Control and Prevention, and Anthony S. Fauci, the head of the National Institute of Allergy and Infectious Diseases. Once regulars at the briefing, neither man was present during Thursday’s affair.

In his initial briefing appearance 10 days ago, Trump floated a payroll tax cut that aimed to restore confidence in the financial markets. The president made brief remarks, then left the podium to Pence, Fauci, Redfield and others in the White House’s task force.

The next day, however, Trump’s economic trial balloon popped on Capitol Hill, where lawmakers have coalesced instead around an emerging plan to send direct payments to Americans. The stock markets continued to fall.

Trump responded by spending more time in the briefing room. His sessions with reporters, broadcast live on cable networks, now regularly last more than an hour, with one this week stretching 83 minutes. He has sought to lead off with major announcements to drive news coverage — but he has spent considerable time answering questions about why those declarations have not met reality.

“I view it in a sense as a wartime president,” Trump told reporters Wednesday in declaring he had invoked the Defense Production Act, a 1950 law aimed at ramping up private industry contributions to the Korean War effort.

By Thursday, a reporter asked the president why he had not yet “pulled the trigger” and ordered corporations to act.

“First of all, governors are supposed to be doing a lot of this work, and they are doing a lot of this work,” Trump said. “The federal government's not supposed to be out there buying vast amounts of items and then shipping. You know, we’re not a shipping clerk.”

On the antiviral drugs, including a product called remdesivir, Trump said his administration was moving to approve the products for coronavirus treatment “as fast as it can possibly be done.”

“And that drug also has been approved, or very close to approved, in that case, by the FDA,” Trump said of remdesivir.

But Hahn later clarified that the product “is going through the normal process” of FDA clinical trials to collect data on its effectiveness and safety.

“So it is those data that are going to inform the decisions that are ultimately made,” Hahn said.