WASHINGTON — President Donald Trump, eager to prove he is healthy and energetic despite his recent hospitalization for COVID-19, returned to the campaign trail Monday night in Florida, speaking for just over an hour in a state that his advisers think he must win in November, but where voters were overwhelmingly repelled by his performance in the first general election debate.

Trump, whose voice sounded hoarse and strained as he began to speak onstage at a hangar at Orlando Sanford International Airport, claimed he was fully recovered and therefore immune to the coronavirus — a claim for which there is no conclusive scientific backing.

“I feel so powerful,” said the president, who did not wear a mask while boarding Air Force One at Joint Base Andrews before leaving Washington. “I’ll kiss everyone in that audience. I’ll kiss the guys and the beautiful women. Just give you a big fat kiss.”

Trump, whose response to a virus that has killed nearly 215,000 Americans remains the biggest threat to his reelection, claimed without any evidence that his Democratic rival, former Vice President Joe Biden, would delay a vaccine and “prolong the pandemic.” He made fun of the small and socially distanced campaign events that Biden has been hosting, and he commended his own campaign for the large crowds it has been turning out at rallies, calling them “the real polls.”

Trump arrived in Florida only hours after the White House physician, Dr. Sean Conley, said that the president had tested negative “on consecutive days” using a rapid antigen coronavirus test not intended for that purpose. Experts cautioned that the test’s accuracy had not been investigated enough to be sure that the president was virus-free or, as his doctor claimed, “not infectious to others.”

In a previous note, Conley had said that the president had a polymerase chain reaction, or PCR, test, which amplifies coronavirus genetic material and is more precise, but he did not release the specific results of that test.

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Many rallygoers on Monday evening did not wear masks, including some of those chosen to stand behind the president’s podium and within the camera shot. And even as Trump claimed he was immune to the virus, White House officials traveling with him acknowledged the risk to those around him. En route to Florida, Mark Meadows, the chief of staff, uncharacteristically wearing a mask, visited the press cabin on Air Force One to thank reporters for covering the event.

Onstage, Trump also mocked questions about whether he would agree to a peaceful transfer of power if he lost. He claimed, falsely, that President Barack Obama had spied on his 2016 campaign and noted, “We’ll take care of it after the election,” adding that it “gives you another reason to go out and vote.”

For the most part, however, the president was back to delivering his regular, factually challenged campaign stump speech, in which he brags about killing terrorists and building a wall along the southwestern border, and accuses the news media of being “frauds.” On Monday night, he boasted about being nominated for the Nobel Peace Prize, and he blamed the news media for not giving him enough credit for a nomination while news outlets had covered Obama being awarded the prize in 2009.

“I turned on the fake news, story after story, they talk about your weather in the Panhandle,” he said, referring to hurricane coverage.

Supporters and critics of the president were likely to focus as much on how the president looked and sounded at the rally as on anything he said. The question of whether he will remain healthy enough to make it through a full Trumpian performance — usually about 90 minutes — without flagging is one that will likely hover over him until the election on Nov. 3. On Monday night, he appeared virtually back to himself despite a scratchy-sounding voice, clocking in at 65 minutes.

On Saturday at the White House, in his first public appearance since returning from Walter Reed National Military Medical Center, Trump spoke for only 18 minutes, uncharacteristically cutting an appearance short. His aides had said he would speak for 30 minutes. But top campaign officials said Monday that they had no concerns about his energy or overall health, only enthusiasm that the candidate was ready to return to the campaign trail.

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“This morning, in our morning conversation, he was getting on my case for not having enough rallies,” said Jason Miller, the Trump campaign’s senior strategist. He said the president’s schedule would include “two to three events a day, and that will grow as we get closer to Election Day.”

The frenetic pace serves as a reminder that with three weeks left in the race, Trump is running from behind. His polling numbers with seniors, a crucial constituency that has been disproportionately harmed by the coronavirus, have been flagging. His campaign has sought to stabilize its numbers with a new television advertisement aimed at changing the negative perception of Trump’s handling of the pandemic, quoting Dr. Anthony Fauci, the nation’s top infectious disease expert, seemingly praising the president.

“I can’t imagine that anybody could be doing more,” Fauci said in a clip that he has since said was taken out of context and used without his consent. The campaign said it intended to continue airing the commercial.

Showing stamina while returning to a hectic campaign schedule less than two weeks after testing positive would help the president continue to play down the virus, just as he has done since March. And it is critical to the case that he has tried to mount against Biden, whom Trump has tried to portray as mentally and physically frail, unable to leave his basement or draw big, enthusiastic crowds.

But over the previous 10 days, it has been the president confined to the hospital and then the White House, while Biden had been out campaigning.

Trump and his advisers are making a robust effort to win Florida’s 29 electoral votes, as he did in 2016, making it clear he was on track for the presidency. In the state, where seniors are a crucial part of the president’s base, his advisers have tried to make up for any losses by adding to his margins of support among Latino and Black voters.

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On a call with reporters on Monday before the rally, Bill Stepien, the president’s campaign manager, said that “every campaign changes from election to reelection” and that any losses with older voters would be “offset by gains in certain voting populations,” listing Black and Hispanic voters.

But the Trump campaign is still making a play for older voters. A recent advertisement juxtaposes Trump’s image against Biden’s and asks, “Who’s better for seniors?” And last week, the president posted a video on Twitter with a message in which he labeled seniors “MY FAVORITE PEOPLE IN THE WORLD!”

Trump’s advisers, who say the public polls are not accurate, hope that he can maintain a robust campaign trail presence between now and the next debate, scheduled for Oct. 22, to try to regain his standing.

Shoring up core constituencies is not where his advisers thought they would be at this point in the election cycle. The president began the year with advisers talking about expanding his political map into Democratic-leaning states like Nevada, Minnesota, New Hampshire and even New Mexico. But this week, Trump is so far scheduled to go to states he won in 2016 and where he is mostly playing defense right now — not just Florida, but also states like Pennsylvania and Iowa.

He seems determined to run the same playbook he ran in the final few weeks of the 2016 race, outworking his opponent on the campaign trail. However, back then, he paired a dizzying schedule of rallies with a trimming back of incendiary tweets. But as an unpopular incumbent through most of his time in office, it is not clear how seeing more of Trump now will necessarily win back support from voters who have tired of his behavior.

“At the end of the 2016 campaign, the president apparently cut back on his Twitter messages and became somewhat less visible at the time the Comey letter was becoming more and more prominent,” said Whit Ayres, a Republican pollster, referring to a letter that the director of the FBI at the time, James B. Comey, sent to Congress after discovering a tranche of Hillary Clinton’s emails on a laptop used by Anthony D. Weiner, the estranged husband of one of her top aides.

That letter is widely seen as affecting voter sentiment about Clinton, the Democratic nominee that year, late in the campaign, and no similar event has so far hurt Biden. Trump, meanwhile, has told aides he wants to be on the campaign trail every day and he has continued with his slashing, acerbic tweets.

“It’s not clear that seeing more of the president is necessarily a help to his campaign,” Ayres said.