Bowing to threats posed by the coronavirus, President Trump reversed course on Thursday and canceled the portion of the Republican National Convention to be held in Jacksonville, Fla., just weeks after he moved the event from North Carolina because state officials wanted the party to take health precautions there.

The surprise announcement threw one of the tent-pole moments of Mr. Trump’s re-election effort into limbo, with the president describing in vague terms how the Republicans would hold his renomination in North Carolina and do “other things with tele-rallies and online.” It was an ill-defined sketch of an August week that Mr. Trump once envisioned drawing huge crowds and energizing his struggling bid for a second term.

While Mr. Trump has spent weeks urging Florida and other states to reopen their economies and return to life as normal, virus cases have surged in Jacksonville and across the region. The president had insisted on moving ahead with the event until Thursday, talking up the big party that Republicans would hold in Jacksonville even with the dangers of large gatherings and some G.O.P. leaders saying they would not attend.

“We won’t do a big, crowded convention, per se — it’s not the right time for that,” Mr. Trump said during a short news conference in the White House briefing room, his third this week, as his aides try to persuade the president to focus on treating the virus seriously in his public comments.

The convention efforts in both Jacksonville and Charlotte, N.C., which have preoccupied some G.O.P. officials and donors for months, now stand as an object lesson in chaotic planning for a party that prizes its ability to raise money and execute splashy displays.

The Jacksonville convention host committee had about $6 million in various accounts, and had spent some of that money already. It had $20 million in commitments that were still firm on Tuesday, according to two officials involved in the fund-raising. On Thursday, they were still assessing whether donors would be able to get their money back but assumed they would not be able to do so in full.

Dan Eberhart, a Republican donor, said that many donors didn’t want to give because they believed the event wouldn’t happen. Edward E. Burr, a real estate developer and member of the Jacksonville host committee, said that in the past few days donors had been calling him expressing serious concerns about the rising number of infections in Florida.

“I’m glad Donald Trump took his head out of the sand long enough to realize what a predictable, preventable disaster he was about to inflict on the city of Jacksonville,” Terrie Rizzo, the chairwoman of the Florida Democratic Party, said in a statement. “His ego-driven political stunt has wasted precious time and resources during a pandemic.”

Mr. Trump claimed that his political advisers had tried to tell him they could make the convention work in Jacksonville, noting the “enthusiasm” that was building. Florida is crucial to Mr. Trump’s re-election prospects, and he particularly needs support from older people — a population that is more vulnerable to the virus and makes up a large share of the state’s voters — to prevail there on Election Day against Joseph R. Biden Jr., who is leading in most polls in the state.

But the president tried to portray himself as more concerned about public health. “I said, ‘There’s nothing more important in our country than keeping our people safe,’” he said of his conversations with advisers. “I just felt it was wrong” to have people “going to what turned out to be a hot spot.”

The president’s sudden focus on health concerns Thursday came after months of playing down the virus. He predicted only three weeks ago that it would “just disappear,” and pushed party officials to proceed with convention plans despite the alarming spike in virus cases in Florida this month. As of this week, Republican officials were still meeting in the state to make the convention a reality.

But as cases surged, voters, donors and elected officials from both parties expressed skepticism about holding a big gathering just several weeks away. A Quinnipiac University poll released on Thursday showed that 62 percent of the state’s voters thought the convention would be unsafe to hold. The poll also showed Mr. Biden leading Mr. Trump by 13 percentage points in Florida; in a Quinnipiac poll in April, Mr. Biden had a four-point lead.

Mr. Trump said that the decision to cancel the event was designed for “safety” and that the news media would have faulted him if he had continued. He said the party might hold rallies that people could join by telephone or video, adding that the actual work of the convention — approving the platform, for instance — would take place in Charlotte, the original site of the gathering.

Mr. Trump’s news conference on Thursday was intended to be about a plan for reopening schools in the fall. But he decided to announce his convention move, overshadowing the other news.

The decision came after some of his advisers pleaded with him to start taking the coronavirus more seriously, to try to revive re-election prospects that have cratered over voter dissatisfaction with his handling of the public health threat. The president held conversations with his new campaign manager, Bill Stepien, and the Republican National Committee chairwoman, Ronna McDaniel, according to people familiar with the discussions.

The decision was made in the last 24 hours, the people said. The discussions were so closely held that some people involved in convention planning learned about the decision from Mr. Trump’s news conference.

The president’s advisers have urged him to treat the virus as if he were a governor overseeing the threat of a hurricane — offering gravitas and taking the situation seriously, but assuring voters that the storm will pass.

Until this week, Mr. Trump’s performance had been exactly the opposite.

In early June, Mr. Trump forced the Republican National Committee to walk away from Charlotte because North Carolina’s governor, Roy Cooper, a Democrat, would not guarantee him that there would be no health restrictions. That potentially meant the president would not get the type of adoring crowds he would have had before the virus struck.

Aides to Mr. Cooper strenuously denied that he had done anything other than ask Republicans for a plan to keep people safe, and said that he had suggested a scaled-down event. “We can’t do social distancing,” Mr. Trump told Mr. Cooper in one of their calls, according to people briefed on the discussion, which the R.N.C. followed up with a letter demanding a full convention.

Democrats have announced they are drastically cutting back their convention in Milwaukee, Wis., over safety concerns, and said last month they would hold an almost entirely virtual convention. (Mr. Trump’s private company moved to trademark the word “tele-rally” in a filing last week.)

But Mr. Trump was adamant about having his celebration. He turned to a Republican ally, Gov. Ron DeSantis of Florida, who had rallied around Mr. Trump’s push to reopen states, to host the convention he craved.

Still, officials repeatedly warned Mr. Trump that the outbreak was getting worse. In Florida, there were 10,239 new cases on Thursday. There were a reported 173 deaths, a record number.

State Senator Joe Gruters of Sarasota, the chairman of the Republican Party of Florida, said he had been looking forward to hosting the convention but understood Mr. Trump’s decision.

“The president is obviously putting safety first and foremost, and I’m glad,” said Mr. Gruters, who had been working on convention planning earlier on Thursday before learning about Mr. Trump’s decision from the afternoon news conference. “We’re disappointed that this is not coming to Jacksonville, but Florida still loves the president, and we’re going to deliver the state.”

But Ron Klain, a former adviser to Mr. Biden who oversaw the Obama administration’s response to Ebola, said Mr. Trump’s mockery of science throughout much of the pandemic had finally caught up with him.

“I think it’s emblematic of why we’re in this mess,” Mr. Klain said. “This convention reversal really fits in with the mistakes he made in May and June, when he belittled anyone who tried to learn the lessons of the Northeast.”

Mr. Trump “went on the attack on those people,” Mr. Klain said, “and now we are paying the price.”

Mr. Trump made his announcement a day after the City Council drafted an emergency ordinance that would have created a convention zone and an area for protesters, and dealt with extended hours for alcohol sales, music and fireworks, among other things. But those measures were not certain to pass, given concerns raised earlier in the week by Mike Williams, the sheriff of Duval County — which encompasses Jacksonville — about insufficient resources for public safety.

“We can’t pull it off,” Mr. Williams told reporters.

Among those pleased with the president’s decision was W.C. Gentry, a Republican lawyer in Jacksonville who had filed a lawsuit on behalf of a downtown church and group of small business owners trying to stop the convention.

“This is great news for our city,” he said. “It would have created the largest super-spreader event in history.”

Opposition to the convention began to cascade this week, Mr. Gentry said, after Sheriff Williams issued his warning and the City Council president, Tommy Hazouri, raised concerns about how much the event would cost Jacksonville.

Mark Meadows, the White House chief of staff, spoke with Sheriff Williams on Tuesday. The security problems, ultimately, were taking too long to resolve. And coupled with the rise in infections, the environment became too difficult.

“It’s the right decision,” said Brian Ballard, the top Republican lobbyist in Florida and a major party donor, who was one of the key fund-raisers for the convention. “The president made a smart decision based on the science and safety, not only of Floridians but of people around the country.”

Mr. Gentry was blunt about the daunting challenges Republicans faced.

“No one in their right mind would come here,” Mr. Gentry said. “This thing was a nightmare for our community.”

“Everything was clearly coalescing to bring this thing to a halt,” he added, “and I think that they chose the better course.”