President Trump returned to the campaign trail Monday, holding his first rally since being hospitalized earlier this month, as part of an intense effort to demonstrate that his bout with covid-19 is behind him and that he is the more vigorous of the two septuagenarian candidates vying for the presidency.

Yet Trump’s rally in Sanford, Fla., came amid concerns that his plans to barnstorm the country could put him and others at risk.

Though Trump has declared himself now “immune” to the virus — which has killed more than 214,000 Americans and infiltrated the White House — he and his team have not clarified for the public the last time he tested negative before his covid-19 diagnosis was announced Oct. 2. This has raised questions about whom Trump may have infected before isolating himself at the White House and then at Walter Reed National Military Medical Center.

On Monday afternoon, however, Trump’s doctor, Sean P. Conley, said in a memo released by the White House that the president had tested negative for the virus “on consecutive days,” using the Abbott rapid testing machine, and was no longer contagious.

The Abbott antigen test produces quick results but has a greater chance of false negatives than the more reliable polymerase chain reaction, or PCR, test. Conley said other diagnostic factors were considered when determining that the president did not pose a threat to others.

“This comprehensive data, in concert with the CDC’s guidelines for removal of transmission-based precautions, have informed our medical team’s assessment that the President is not infectious to others,” he wrote in the memo released to the public.

Some of Trump’s aides and associates initially hoped that his coronavirus diagnosis would help focus him on the pandemic, allowing him to emerge as a sympathetic figure with a newfound sense of seriousness and empathy.

That, so far, has not happened.

“The cure cannot be worse than the problem itself can. The cure cannot be worse,” Trump told the Sanford crowd — many of whom were not wearing masks — referring to public health restrictions in many states. “But if you don’t feel good about, if you want to stay, stay relaxed, stay. But if you want to get out there, get out. One thing with me, the nice part, I went through it. Now they say I’m immune . . . I feel so powerful.”

Since contracting the virus, Trump has remained dismissive of the threat posed by the pandemic, reappearing in public seemingly invigorated by his survival. He has doubled down on his push for reopening the country while continuing to discount social distancing and other public health practices.

Trump, 74, trails Democratic rival Joe Biden, 77, in national and key state polls, with voters giving the president poor ratings for his handling of the pandemic.

His return to the campaign trail, with back-to-back-to-back rallies at least through Wednesday, is being driven by Trump himself, according to aides, and his schedule so far reflects the frenetic energy of a man trying to outrun both a deadly illness and an electoral defeat.

After rallies the first half of the week in Florida, Pennsylvania and Iowa, Trump is expected to return to Florida on Thursday and Friday, as well as hold more rallies over the weekend, probably in Ohio and Wisconsin, said a senior campaign adviser, who, like others, spoke on the condition of anonymity to describe plans that have not been made public. The president also plans to travel soon to North Carolina, the adviser said.

Two senior campaign officials said the operation needs more money to run TV ads, is playing defense in many states, and must win Pennsylvania and North Carolina — both uphill fights. Campaign manager Bill Stepien has told others that they need to be careful with money because there are so many states where the campaign needs it, including many that Trump won in 2016, officials said.

As the president returned to the trail Monday, Biden kicked off a week of events in several states that Trump captured in 2016 but that Democrats say they can win this year. Biden campaigned Monday in Ohio, a state Trump won easily four years ago, and will travel Tuesday to Florida, where polls show a tight race.

Biden campaign officials are eager to renew their efforts to draw a sharp contrast with Trump on the economy and the pandemic.

And they intend this week to reemphasize the Democrat’s working-class upbringing as part of what they call the “Scranton vs. Park Avenue and Wall Street” comparison they are making with Trump, an aide said. These themes were evident as Biden campaigned Monday in Toledo, where he accused Trump of acting in a rash manner since contracting the coronavirus and re-upped his attacks on the president’s failure to contain the pandemic.

“His reckless personal conduct since his diagnosis has been unconscionable,” Biden said. “The longer Donald Trump is president, the more reckless he seems to get.”

Biden tested negative Monday for the coronavirus, his campaign said. It was the seventh known test he has taken since Oct. 2, the day Trump announced his diagnosis. All seven have been negative, and the more accurate PCR test has been used, according to the campaign.

There continues to be criticism that the White House is not releasing enough information about Trump’s health to assure the public about his well-being.

Joshua Sharfstein, a physician at the Johns Hopkins Bloomberg School of Public Health, said that the president might not be contagious but that it was difficult to reach any conclusion based on the Conley memo Monday. He said there were “a couple of actual test results, then an interpretation of other test results” whose contents were not disclosed. He said that the antigen test “is probably the least accurate.”

“The challenge is that the president’s physician is not a credible source,” Sharfstein said. “It’s very clear that he’s taken ‘accentuate the positive’ to heart. For credibility, you need to release everything or have a totally credible person review everything.”

On a call with reporters Monday morning, top campaign aides emphasized Trump’s eagerness to return to public campaigning and touted what they claimed was his robust physical health.

“He is strong, he is energetic, he is raring to go, and I think his campaign calendar reflects his health and well-being and enthusiasm to get back on the trail,” said Stepien, who added that Trump was the campaign’s “best asset” and would be “a big shot in the arm of the campaign.” Stepien also tested positive for the coronavirus earlier this month.

Senior adviser Jason Miller said he expects that the president will start doing two to three events a day, before ramping up to as many as half a dozen in the campaign’s final stretch.

“You’re going to see President Trump flat-out outworking Joe Biden down the home stretch here, just as he has shown in his previous campaign,” Miller added. 

At his last public rally — in Duluth, Minn. — before his covid-19 diagnosis, the normally verbose Trump kept his remarks uncharacteristically brief, speaking for just 45 minutes. And Saturday, at an event on the White House South Lawn aimed at conservative activists, the president again offered a curtailed speech, talking for 18 minutes, with bandages — probably from his covid-19 treatment — visible on his hands. On Monday night, he spoke for about an hour.

Asked whether the campaign is taking any additional precautions to safeguard Trump’s health as he heads back out in public, a spokesman deferred to the White House. In an emailed statement, White House spokesman Judd Deere said Conley will “closely monitor” Trump.

“As President Trump’s physician has laid out, the president has responded extremely well to his treatments to defeat Covid-19,” Deere wrote.

But Monica Gandhi, an infectious-disease expert and professor of medicine at the University of California at San Francisco, said she would counsel caution with regard to the ramping up of Trump’s activities and perhaps a more moderate campaign schedule, at least to start.

“This is not something I would recommend as an infectious-disease doctor for someone who nominally had covid but had severe enough symptoms to be hospitalized with some oxygenation at one point,” Gandhi said. “I am concerned about such a busy schedule right afterwards. People do definitely report a lot of fatigue, people report still feeling short of breath and having a persistent cough, and there are even longer-term symptoms.”

Gandhi added that Trump’s desire to start vigorous campaigning is “unusual to say the least” and that she would “advise him to rest for about a week more, at least.”

She also said she would encourage Trump to remain socially distanced and to wear a mask while on his flights — something he did not do Monday as he boarded Air Force One en route to Florida — given the heightened risk of contracting other viral diseases, such as the flu.

Days before Trump caught the virus, he joked with donors at a fundraiser at his hotel in Washington that his aides were making him do too many events. Now, however, Trump has pushed his team to hold several events a day, fearful he is behind in the polls with just three weeks until Election Day.

The president and his allies have seen grim polling, which shows him trailing in many battleground states that he won in 2016, including Michigan, North Carolina and Pennsylvania, advisers said. The latter two are particularly crucial to Trump’s path to victory. 

The president has asked allies whether his campaign advisers are doing a good job and has complained in recent days about being outspent on television by Biden, officials said.

Trump’s argument, one adviser said, is that the campaign is simply a series of rallies and television appearances and that to win, he must dominate the airwaves and the stage at almost all times.

Inside the West Wing, two officials said, some expressed serious concern about Trump’s South Lawn event Saturday, where the president addressed a crowd of hundreds from a balcony. But Jared Kushner — Trump’s son-in-law and senior adviser, who is close with Candace Owens, one of the event’s organizers — and White House Chief of Staff Mark Meadows, who is less worried about the coronavirus, pushed to hold the event. Since Trump returned from the hospital on Oct. 5, White House aides have mostly been avoiding the Oval Office, a senior administration official said. Meadows and senior adviser Dan Scavino are the two aides who have spent the most time with the president since his diagnosis late last month, this official added.

The subject of how to handle the pandemic will continue to be front and center in the race.

On Thursday — originally the date of the second presidential debate, which has been canceled — Biden will participate in a town hall in Philadelphia hosted by ABC News. Trump is also expected to participate in a town hall of his own Thursday.

Biden has made a point of emphasizing his own adherence to safety vs. Trump’s habit of flouting the advice of public heath experts. As he walked to the lectern Monday, Biden wore a protective mask, though he removed it when he started speaking.

The White House, meanwhile, has more formally embraced a strategy of “focused protection” pushed by a group of epidemiologists from Oxford, Harvard and Stanford. It calls for allowing most Americans to return to normalcy while protecting vulnerable populations. Critics have called it an endorsement of the deadly “herd immunity” approach.

Trump pushed a similar message during a call with supporters Sunday, saying 99.9 percent of young people who catch the virus survive.

“But for people that are a little bit older and not 100 percent perfectly well, if they have diabetes, if they have heart problems, if they have any of those problems, it gets to be quite bad,” Trump said, before promising soon-to-come cures for those vulnerable populations.

With less than a month until Election Day, it is unclear exactly what Trump’s closing message will be. Campaign aides and White House officials have expressed eagerness to keep the focus on rebuilding the economy, as well as optimism that the fight to seat Amy Coney Barrett, Trump’s Supreme Court nominee, will help galvanize Republican voters.

But Trump so far has shown no message discipline beyond the usual scattershot of accusations, false claims and boasts that have dominated his Twitter feed in recent days.

Doug Deason, a major Trump donor in Texas, said he believes that the president is trending up in several key states — Florida, Ohio, Pennsylvania and Michigan — and that Biden is “totally irrelevant,” putting the president’s electoral fate in his own hands.

“This race is Trump vs. Trump,” Deason said. “He is the only one who can beat himself.”

Steven Mufson contributed to this report.