On a Saturday in September 2020, with Covid-19 killing more than 600 Americans daily and hundreds of thousands of deaths still to come, Dr. Deborah L. Birx, a member of the White House coronavirus task force, heard her cellphone ring. It was Dr. Scott Rivkees, the Florida surgeon general. He was distraught.

“‘You won’t believe what happened,’” she said he told her. Months before Covid vaccines would become available, Gov. Ron DeSantis had decided that the worst was over for Florida, he said. Mr. DeSantis had begun listening to doctors who believed the virus’s threat was overstated, and he no longer supported preventive measures like limiting indoor dining.

Mr. DeSantis was going his own way on Covid.

Nearly three years later, the governor now presents his Covid strategy not only as his biggest accomplishment, but as the foundation for his presidential campaign. Mr. DeSantis argues that “Florida got it right” because he was willing to stand up for the rights of individuals despite pressure from health “bureaucrats.” On the campaign trail, he says liberal bastions like New York and California needlessly traded away freedoms while Florida preserved jobs, in-person schooling and quality of life.

But a close review by The New York Times of Florida’s pandemic response, including a new analysis of the data on deaths, hospitalizations and vaccination rates in the state, suggests that Mr. DeSantis’s account of his record leaves much out.

As he notes at most campaign stops, he moved quickly to get students back in the classroom, even as many of the nation’s school districts were still in remote learning. National research has suggested there was less learning loss in school districts with more in-person instruction.

Some other policies remain a matter of intense debate. Mr. DeSantis’s push to swiftly reopen businesses helped employment rebound, but also likely contributed to the spread of infections.

But on the single factor that those experts say mattered most in fighting Covid — widespread vaccinations — Mr. DeSantis’s approach proved deeply flawed. While the governor personally crusaded for Floridians 65 and older to get shots, he laid off once younger age groups became eligible.

Tapping into suspicion of public health authorities, which the Republican right was fanning, he effectively stopped preaching the virtues of Covid vaccines. Instead, he emphasized his opposition to requiring anyone to get shots, from hospital workers to cruise ship guests.

While Florida was an early leader in the share of over-65 residents who were vaccinated, it had fallen to the middle of the pack by the end of July 2021. When it came to younger residents, Florida lagged behind the national average in every age group.

That left the state particularly vulnerable when the Delta variant hit that month. Floridians died at a higher rate, adjusted for age, than residents of almost any other state during the Delta wave, according to the Times analysis. With less than 7 percent of the nation’s population, Florida accounted for 14 percent of deaths between the start of July and the end of October.

Of the 23,000 Floridians who died, 9,000 were younger than 65. Despite the governor’s insistence at the time that “our entire vulnerable population has basically been vaccinated,” a vast majority of the 23,000 were either unvaccinated or had not yet completed the two-dose regimen.

A high vaccination rate was especially important in Florida, which trails only Maine in the share of residents 65 and older. By the end of July, Florida had vaccinated about 60 percent of adults, just shy of the national average. Had it reached a vaccination rate of 74 percent — the average for five New England states at the time — it could have prevented more than 16,000 deaths and more than 61,000 hospitalizations that summer, according to a study published in the medical journal The Lancet.

Florida’s spike in deaths subsided that fall, as it did elsewhere. Overall, the state’s death rate during the pandemic, adjusted for age, ended up better than the national average. Some public health experts credit the state’s robust health system and strong performance in the pandemic’s first year or so.

But in Florida, unlike the nation as a whole — and states like New York and California that Mr. DeSantis likes to single out — most people who died from Covid died after vaccines became available to all adults, not before. As the governor’s political positions began to shift, so did his state’s death rate, for the worse.

Mr. DeSantis and his aides have said that his opposition was to mandates, not to the vaccinations themselves. They say the governor only questioned the efficacy of the shots once it became evident that they did not necessarily prevent infection — which prompted him to criticize experts and the federal government.

Mr. Desantis’s office did not respond to specific questions about his Covid response. After publication, a spokesman, Jeremy Redfern, sent a statement criticizing the federal government for “pseudoscientific policies” and claiming the governor had protected Floridians’ “right to make their own health decisions.”

But for some with a close-up view of Covid in Florida, the Delta wave’s toll was evidence of the insular leadership style that Mr. DeSantis has also displayed in his struggling presidential campaign. He boasted of standing up to health experts, but carefully tended to his base of political supporters. Tapping into the Republican revolt against scientific authority made him a political star. But that revolt came with costs.

“These were preventable deaths,” Dr. Rivkees, who resigned as Florida’s surgeon general in September 2021, said in a recent interview. “It breaks my heart thinking that things could have turned out differently if people embraced vaccines instead of this anti-vax stuff.”

Mr. DeSantis entered the pandemic a cautious pragmatist, mostly embracing the scientific consensus on prevention measures. But the governor, who often describes himself as a “data guy,” also personally pored over scientific research.

He soon assembled his own kitchen cabinet of pandemic advisers. Pushing away Dr. Rivkees and Dr. Birx, he bonded with academics who reinforced his thinking that older people and others who were vulnerable should be protected from infection, but everyone else should be allowed to lead normal lives.

Dr. Jay Bhattacharya, a Stanford University health policy expert, said that an aide to Mr. DeSantis called him out of the blue in the summer of 2020, saying the governor wanted to confer about reopening schools.

“I cited a whole bunch of papers in our conversation,” Dr. Bhattacharya recalled in a 2020 interview. “It was clear he had already read all of them.”

Every night, Mr. DeSantis’s staff in Tallahassee assembled a binder stuffed with documents and delivered it to the governor’s mansion by 4 a.m. He read it while exercising and gave his chief of staff instructions to relay at a 7 a.m. staff meeting.

The governor had early success in following his instincts. In 2020, the state supplied its nearly 4,000 long-term care homes with Covid tests and isolated Covid patients, avoiding New York’s mistake of releasing Covid patients from hospitals to nursing homes where they infected others. Florida’s death rate in the pandemic’s first year, adjusted for age, was lower than all but 10 other states’.

Florida was also one of only four states to require schools to hold in-person classes in the fall of 2020, a move that Mr. DeSantis has said defied the nation’s public health experts. In fact, Dr. Anthony S. Fauci, a federal infectious disease expert on former President Donald J. Trump’s task force, said repeatedly that summer and fall that schools could open safely with the right precautions. Nonetheless, facing strong opposition from teachers’ unions, nearly three-fourths of the nation’s 100 largest school districts offered only remote learning that fall.

At the same time, though, the governor was embracing more extreme views, including those of Dr. Scott W. Atlas, a Stanford neuroradiologist with no expertise in infectious diseases. Dr. Atlas was a frequent commentator on Fox News when Mr. Trump named him to his Covid task force in August 2020.

Both he and Dr. Bhattacharya argued that people who were not at risk of severe consequences should not face Covid restrictions. If they were infected, they would develop natural immunity, which would eventually build up in the population and cause the virus to fade away, they said.

Many public health experts were alarmed by this strategy, which was articulated in a document known as the Great Barrington Declaration. They said it would be impossible to ring-fence the vulnerable, or even to clearly communicate to the public who they were. Besides older Americans, as many as 41 million younger adults were considered to be at high risk of severe disease if infected because of underlying medical conditions like obesity.

Dr. Atlas, however, argued that the virus was not dangerous to an overwhelming majority of Americans. Both he and Dr. Bhattacharya said the Covid death rate for everyone under 70 was very low. Dr. Atlas claimed that children had “virtually zero” risk of death. Neither man responded to requests for comment.

As of this summer, more than 345,000 Americans under 70 have died of the virus, and more than 3.5 million have been hospitalized with Covid. The disease has killed nearly 2,300 children and adolescents, and nearly 200,000 have been hospitalized.

Other members of the White House task force, including Dr. Birx, fought to keep Dr. Atlas out of public view, calling his views dangerous.

But Mr. DeSantis gave him a platform at a series of public events in Florida at the end of the summer of 2020. He would go on to echo Dr. Atlas’s views, sometimes in modified form, throughout the pandemic.

Disturbed by Dr. Atlas’s influence, Dr. Rivkees called Dr. Birx on Sept. 19, 2020. “I was very concerned about the let-’er-rip philosophy espoused by Dr. Atlas,” he said.

As soon as they hung up, Dr. Birx said, she texted Jared Kushner, Mr. Trump’s son-in-law and another White House task force member, asking him to stop Dr. Atlas from spreading his message in other states.

“This is going to drive up hospitalizations and deaths,” she said she told Mr. Kushner.

Days later, Mr. DeSantis issued the first in what became a barrage of edicts reining in virus mitigation measures. He had found his political lane.

“When 2020 got underway, I was merely a state governor entering his second year in office,” he wrote in his 2023 book, “The Courage to Be Free.” “Within six months, I would emerge as one of the leading anti-lockdown elected officials in the world.”

Mr. DeSantis was waiting at Tampa General Hospital when one of the earliest shipments of Covid vaccine arrived on Dec. 14, 2020. “I had also the privilege to be able to actually sign for the vaccines from FedEx,” he said that day. When a nurse received the hospital’s first vaccine a few minutes later, Mr. DeSantis cheered, “Yay!”

Mr. DeSantis subsequently promoted the shots in 27 counties. Florida offered the vaccine to everyone 65 and older, an eligibility system simpler than an early one recommended by the Centers for Disease Control and Prevention, and adopted by many states, that prioritized essential workers and those over 75.

In February 2021, Mr. DeSantis urged officials at a news conference in Hernando County — a largely rural, heavily Republican area north of Tampa that was lagging in vaccinating older people — to “get those numbers up.” If older people were not lining up for shots, he said, bring the vaccine closer so they could simply hop in their golf carts to get it.

But his enthusiasm for shots waned fast, tracking the growing hostility toward them among the party’s conservative activists. In late February, when Mr. DeSantis hosted a gathering of such activists for the Conservative Political Action Conference in Orlando, he boasted that Florida was an “oasis of freedom” in a nation led by misguided health authorities.

By the time all adults became eligible for the vaccines in April of that year, Mr. DeSantis was rarely promoting them.

“Some are choosing not to take it, which is fine,” he said in March, at a 100-minute public event on Covid in which he did not once urge people to get vaccinated. In dozens of appearances on Fox News in the first half of 2021, he was carefully neutral about shots, except for those over 65.

“Younger people are just simply at very little risk for this,” Mr. DeSantis said on a prime-time show on Fox News in April 2021, although tens of thousands of Americans under the age of 50 had already died of Covid.

A few months later, he told Fox News that he had concluded early on that Covid “was something that was risky for elderly people,” but that it posed minimal risks for people “who were in reasonably good health, who were, say, under 50.”

“He knows how to skate the way the puck’s going,” said David Jolly, a former Republican congressman from Florida. “I think he was always torn with the politics of populism, so as soon as he could escape from his leadership on vaccines, he did.”

For health officials on the ground, the shift was clear. Dr. Alina Alonso, who recently retired as the health director for Palm Beach County, said that Mr. DeSantis’s message “switched from ‘Let’s get everybody vaccinated 65 and older’ to ‘Vaccines are not really useful.’”

The pivot mattered because “there are people in this state who will do what he says,” said Dan Gelber, the Democratic mayor of Miami Beach. “He’s a popular governor.”

The data-driven governor also turned away from Covid case data. Two former aides who spoke on the condition of anonymity for fear of damaging their careers said that DeSantis staff members complained to Jared Moskowitz, then the state’s head of emergency management, that more tests detected more infections, which spawned bad press.

In May 2021, Florida closed its 27 state-run testing centers. The next month, on orders from the governor’s office, the Health Department halted daily reports on infections and deaths, switching to weekly reports that drew less attention.

The governor also began to attack Dr. Fauci and other federal pandemic experts. A political fund-raising operation backing his re-election began that July to hawk $12 beer bottle sleeves and $9 T-shirts carrying the slogan “Don’t Fauci My Florida.”

Both polls and political events showed that Republicans were not as excited as Democrats about the shots. At an Alabama political rally that August, Mr. Trump recommended the vaccine — and was booed. When a reporter asked Mr. DeSantis later that year if he had gotten a booster shot, he responded that he had gotten “the normal shot.”

After the highly contagious Delta variant began spreading in Florida that summer, Mr. DeSantis insisted that his approach had worked. Younger adults were driving the surge but “they’re not getting really sick from it or anything,” he said, adding: “They will develop immunity as a result of those infections.”

But they were getting sick. And vaccinations, which Mr. DeSantis suddenly began recommending again in late July, took weeks to confer protection. With hospitalizations rising, he began a campaign to offer monoclonal antibody treatments — a triage response to the pandemic’s frightening resurgence.

The drug cost vastly more than shots and required more medical staff to administer. Within about six weeks, the state had administered more than 90,000 treatments and probably kept 5,000 people out of the hospital, Dr. Rivkees said.

Mr. DeSantis accused the media in early August of “lying” about Covid patients’ flooding hospitals. Two weeks later, Mary C. Mayhew, head of the Florida Hospital Association, said: “There can be no question that many Florida hospitals are stretched to their absolute limits.”

Mickey Smith was then the chief executive of Oak Hill, the biggest hospital in Hernando County. As the Delta variant raged through the county that month, he documented the impact on the 346-bed hospital in near-daily staff memos.

The morgue was filled to capacity. Oxygen was in such demand that the supplier would only partly fill Oak Hill’s tank. Ambulances were lined up outside to unload new patients, some of whom had to be shunted to a hastily erected outdoor tent.

“Our patients are younger and sicker,” Mr. Smith wrote. Of 17 patients on ventilators in intensive care on Aug. 13, 2021, more than half were younger than 55. Only one was vaccinated.

“People say that the decision about vaccination is a personal one and it doesn’t affect anyone else,” Mr. Smith wrote. “Tell that to the kids who lost their mom.”

Dr. Rivkees’s successor was Dr. Joseph Ladapo, whom Mr. DeSantis called “the anti-Fauci.” For the rest of the pandemic, the governor took an increasingly extreme stance on Covid vaccines.

He told Fox News in late 2022: “Our medical establishment never wanted to be honest with people about the potential drawbacks.”

When shots became available last year for children under 5, Florida did not preorder them because, Mr. DeSantis said, he did not consider them “appropriate.” Florida’s vaccination rates are well below the national average for children under 5. The state also trails in booster shots.

After Dr. Ladapo issued misleading claims about the risks of Covid shots for young men, the heads of the C.D.C. and the Food and Drug Administration sent a scathing four-page rebuttal. Such misinformation “puts people at risk of death or serious illness,” they said.

But Mr. DeSantis, who won re-election last fall by nearly 20 points, now calls the F.D.A. untrustworthy. Campaigning in New Hampshire last month, he said that the agency, which authorized the vaccines, had been “captured by the pharmaceutical companies.”

He also instigated, with fanfare, a state grand jury investigation into possible “misconduct” by scientists and by Pfizer and Moderna, the vaccine manufacturers. No charges have been brought.

While the pandemic waned, leaving more than 80,000 Floridians and 1.13 million Americans dead, the governor continued to push policies that kept him at the vanguard of the anti-vaccine and anti-mandate conversation. A new state law, signed by Mr. DeSantis in May, bans government agencies, businesses and schools from requiring Covid testing, vaccination or mask wearing.

And despite little evident interest in Covid among Republican voters, he has campaigned for president as a dissenter who will not be silenced.

“Everything involving Covid — I think there needs to be major, major accountability,” he said in Iowa this month. “Because if there’s not, if you don’t have a reckoning, they are going to do it again.”

Ashley Wu,Dana Goldstein, Nicholas Nehamas and Sarah Mervosh contributed reporting. Susan C. Beachy and Kitty Bennett contributed research. Video production by Chevaz Clarke-Williams.