WASHINGTON — President Trump acknowledged on Tuesday that the coronavirus pandemic was growing more severe in the United States and endorsed mask wearing in a shift after weeks of playing down the seriousness of the crisis that has killed more than 140,000 Americans.

Rather than just “embers” of the virus, as he has repeatedly characterized recent outbreaks afflicting much of the country, Mr. Trump conceded that there were now “big fires,” particularly in Florida and elsewhere across the South and West. He vowed to press a “relentless” campaign to curb the spread without offering any new specific plans for how to do so.

“It will probably, unfortunately, get worse before it gets better,” Mr. Trump told reporters as he resumed the televised coronavirus briefings that he had called off in late April. “Something I don’t like saying about things, but that’s the way it is. It’s what we have.”

The president’s shift had its limits, however, as he again congratulated himself on his handling of the pandemic, admitted no missteps and made a number of specious claims. He included none of his public health experts in the briefing and falsely asserted that he had never resisted wearing a mask. And he contradicted his own press secretary, who had told reporters just hours earlier that the president was sometimes tested for the virus multiple times a day; in fact, he said, he has never been tested more than once in a single day.

But Mr. Trump was notably less dismissive about the pandemic, a reflection of the dawning realization within his team that the virus not only is not going away but has badly damaged his standing with the public heading into the election in November. Approval of his handling of the pandemic has fallen from 51 percent in late March to 38 percent last week in polling by The Washington Post and ABC News.

Former Vice President Joseph R. Biden Jr., the presumptive Democratic nominee who now leads Mr. Trump by double digits, has assailed him in recent days for ignoring a devastating threat to the United States.

On Monday, Mr. Biden said the president had “raised the white flag” in the fight against the virus. On Tuesday, he said the incumbent had failed to help working families hurt by the economic collapse.

“You know, he’s quit on you, and he’s quit on this country,” Mr. Biden said as he released a plan for child and elder care. “But this election is not just about him. It’s about us. It’s about you. It’s about what we’ll do, what a president’s supposed to do.”

Mr. Trump’s briefing was more tightly disciplined than the daily performances in March and April when he would talk for more than an hour, picking fights with governors and reporters and making ill-considered remarks like suggesting bleach as a treatment for the coronavirus. On Tuesday, he read from a prepared script, took fewer questions than in the past and ended the session in 27 minutes, shorter than all but one of the 50 briefings he did in the spring, according to Factba.se, which tracks his public appearances.

Advisers have urged him to be less combative, demonstrate more concern over the latest surge in infections and avoid straying into areas that have been counterproductive. Even so, the president wandered far afield when he offered supportive words to Ghislaine Maxwell, who was charged with luring underage girls into the orbit of the financier Jeffrey Epstein, who killed himself in August after he was charged with sex trafficking.

“I’ve met her numerous times over the years, especially since I lived in Palm Beach, and I guess they lived in Palm Beach,” Mr. Trump said. “But I wish her well.”

The White House did not invite to the briefing Dr. Anthony S. Fauci, the government’s top infectious disease expert, who has come under fire from the president and his team. Dr. Deborah L. Birx, the White House coronavirus response coordinator, was not in the room, either.

But Dr. Fauci appeared on CNN an hour before Mr. Trump’s briefing with a message that contradicted the president’s assertions.

While Mr. Trump has boasted of the number of tests conducted to assert that the virus is under control, Dr. Fauci said that was not enough if it took days to get results. “Just the number of tests that you do doesn’t always give you a right reflection of how well things are working or not,” he told Jake Tapper in an interview.

Dr. Fauci also pushed back on the president’s description of him as “a little bit of an alarmist,” as Mr. Trump put it on Fox News over the weekend. “I consider myself more a realist than an alarmist, but people do have their opinions other than that,” Dr. Fauci said mildly.

Mr. Trump again bragged about the “tremendous amount of testing,” but when asked about the delays in results, he agreed that it should be fixed. “If the doctors and the professionals feel that even though we are at a level that nobody ever dreamt possible that they would like to do more, I’m OK with it,” he said.

“Ultimately, our goal is not merely to manage the pandemic but to end it,” he added. “We want to get rid of it as soon as we can.”

Mr. Trump’s decision to resume televised virus briefings came as the number of new cases soared far above what it was when he was last addressed the country about the pandemic on a daily basis. The United States is recording about 60,000 new infections a day, far more than the increase in tests in some states. The number of deaths, after falling substantially, is up 64 percent over the past two weeks.

The president again insisted the virus would “disappear” but conceded that it remained serious. “We have embers and fires, and we have big fires, and unfortunately now Florida is in a little tough or in a big tough position,” he said.

Weeks after claiming that “99 percent” of coronavirus cases were “totally harmless,” the president sounded less sanguine on Tuesday, calling it “a nasty horrible disease,” although he continued to falsely insist that the mortality rate in the United States was among the lowest in the world.

Mr. Trump urged Americans to avoid packed bars and offered his most robust endorsement of masks, saying, “When you can, use a mask,” even as he falsely claimed he had always been supportive. “I have no problem with the masks,” he said, holding up a blue one with a presidential seal. “I view it this way: Anything that potentially can help, and that certainly can potentially help, is a good thing. I have no problem. I carry it. I wear it. You saw me wearing it a number of times, and I’ll continue.”

In fact, Mr. Trump has worn a mask in public on only one occasion — during a recent visit to Walter Reed National Military Medical Center in Maryland. Until then, he often disparaged masks: In April, after public health advisers recommended wearing them, he said, “I don’t think I’m going to be doing it.” Mr. Trump mocked Mr. Biden in May for donning one, calling them “a double-edged sword” and even suggesting that wearing a mask was a political statement against him.

He shifted his stance only after many senior Republicans, including Senator Mitch McConnell of Kentucky, the majority leader, and several governors began promoting them more vigorously. By this week, Mr. Trump began saying that it was “patriotic” to wear a mask, in contrast to his supporters, who have claimed that mandating masks is an infringement on their civil liberties.

But even after posting a Twitter message on Monday urging masks, the president was spotted that night at his Washington hotel mingling with guests without wearing one.

And even as the president sought to recalibrate his message on the virus on Tuesday, he was struggling to reconcile it with the rest of his team. Asked at an earlier news briefing about Mr. Trump’s failure to wear a mask at his hotel, Kayleigh McEnany, the White House press secretary, told reporters that it was not as urgent for the president to wear one since he was tested regularly for the virus.

“The president is the most tested man in America,” she said. “He’s tested more than anyone, multiple times a day. And we believe that he’s acting appropriately.”

Mr. Trump offered a different account from the same lectern. “I do take probably on average a test every two days, three days,” he said. “I don’t know of any time I’ve taken two tests in one day.”