MIAMI — Florida billed itself as a coronavirus oasis when it wooed the Republican National Convention: The pandemic appeared beneath management, and Republican state and metropolis leaders welcomed a serious televised occasion to indicate off the progress and cement votes for President Trump within the nation’s largest presidential battleground.

That pitch collided with the grim actuality of the coronavirus and its explosive surge in Florida when Jacksonville, the conference’s new host, imposed a requirement on Monday that folks put on masks indoors, exactly the mandate that the Republican Social gathering had hoped to keep away from for its celebrations.

The masks order is sort of sure to rankle Mr. Trump, whose demands for a traditional, packed rally compelled the transfer earlier this month to Jacksonville from Charlotte, N.C., within the first place.

The resurgence of the virus in Florida, the place the variety of new circumstances has been rising practically daily, has turned the conference into a fair riskier gamble. In a must-win state for Mr. Trump — and arguably a must-win metropolis for Republicans — current polls have proven that Jacksonville residents are not looking for the conference.

Masks shall be required in indoor public locations and in some other gathering spots the place social distancing isn’t doable, Jacksonville officers stated. It’s unclear how lengthy the masks order shall be in place, however it would presumably apply to the 15,000-seat VyStar Veterans Memorial Enviornment, the place three nights of conference speeches, together with Mr. Trump’s, are scheduled for Aug. 24-27.

The president has refused to put on a masks himself, even because the nation’s prime docs have emphasised that facial coverings are essential to slowing the virus. Vice President Mike Pence has lately worn masks in public, together with over the weekend in Texas, and inspired different folks to do the identical.

In a telling transfer, Mayor Lenny Curry, a Republican, who had lobbied for the conference and resisted a masks requirement, was absent from Monday’s announcement. His spokeswoman stated he had a previous household obligation.

“We’re nonetheless two months out from that occasion,” he stated final week. “Within the meantime, we’ve got to give attention to what’s taking place within the metropolis and what we are able to management.”

His administration’s coverage reversal got here after a morning convention name with native hospitals. Duval County, which incorporates Jacksonville, is now averaging about 384 new cases of the virus a day. Two weeks in the past, that determine was 32.

In an announcement, the Republican Nationwide Committee stated it will comply “with native well being laws in place on the time.”

“The occasion remains to be two months away, and we’re planning to supply well being precautions together with however not restricted to temperature checks, out there P.P.E., aggressive sanitizing protocols, and out there Covid-19 testing,” the assertion stated.

Mr. Trump didn’t instantly reply to the announcement. However as he has pushed states to reopen their economies, he has proven a capability to bend native laws to his will. Officers in Tulsa, Okla., rescinded a curfew ahead of the president’s rally there earlier this month after Mr. Trump stated on Twitter that he had mentioned the curfew with the mayor, a Republican.

Democrats say Republicans wish to use the conference to shore up votes in Northeast Florida.

“Jacksonville — Duval County — is a swing metropolis in a swing state,” stated State Senator Audrey Gibson, the Democratic majority chief, who represents town. “Duval County went blue within the final election. So it’s fairly apparent what that is about.”

In 2016, Mr. Trump received Duval County by a mere 5,968 votes, although Hillary Clinton by no means campaigned there. In 2018, the county voted for Andrew Gillum, a Democrat, for governor, although its surrounding suburbs remained deeply pink.

Jacksonville’s broad tv market contains Republican suburbs like prosperous St. Johns County, residence to St. Augustine — the kind of place the place Democrats hope to win over sufficient white, college-educated girls to stay aggressive in November. Former President Barack Obama received Florida twice partly by retaining vote margins shut in Duval County. (For the newcomers: It’s pronounced “DOO-val.”)

Along with coping with virus this yr, Jacksonville has additionally develop into a microcosm of how the nation has been forced to confront systemic racism within the wake of George Floyd’s killing. African-People make up practically a 3rd of Jacksonville’s inhabitants of 912,000 however have been traditionally excluded from energy in native authorities.

That Jacksonville — a metropolis perennially overshadowed by Miami, Orlando and Tampa that beneath regular circumstances may by no means have held a conference — will get its second within the political highlight whereas it grapples with the virus and the unfulfilled guarantees to its African-American residents has raised the stakes for native leaders. What quantities to an enormous marketing campaign rally for a divisive president might solely result in extra contagion and unrest.

“So far as the R.N.C. is anxious, that took us 100 steps backwards,” stated Moné Holder, senior program director for New Florida Majority, one of many social justice organizations that participated in protests towards police brutality. “The powers that be attempt to paint it as, ‘one metropolis, one Jacksonville’ — I feel that’s the quote the mayor likes to make use of — but it surely’s very, very divided, racially and economically.”

A on-line ballot of two,524 registered Duval County voters launched final week by the Public Opinion Research Lab on the College of North Florida discovered that 58 p.c of respondents opposed internet hosting the conference, with 71 p.c involved that it will result in virus unfold, and 61 p.c nervous that it will lead to unrest. Solely 39 p.c permitted of Mr. Trump’s efficiency in workplace.

However Mr. Trump has an affinity for Florida, his adopted home, and located an keen ally in Gov. Ron DeSantis, a Republican, who is alleged to have ambitions for larger workplace. Mr. DeSantis has talked in regards to the conference’s “large financial affect” as its chief draw.

Even the governor, nevertheless, hedged a bit when requested in regards to the conference over the weekend, saying planning was underway throughout “a dynamic state of affairs.”

“We all the time stated, look, it’s a piece in progress. We’re going to attempt to get to sure,” he stated. “I feel we’re going to be wonderful by then. It’s a few months away.”

On Monday, he maintained that native governments might act on their very own however he wouldn’t impose a statewide masks mandate.

Mr. Curry’s opposition to requiring masks had drawn uncommon public criticism from outstanding members of the group, together with Hugh Greene, who ran the Baptist Well being hospital system for 20 years earlier than retiring final yr. After Monday’s announcement, Mr. Greene stated the conference ought to require facial coverings and social distancing.

“Something that’s good for the residents of Jacksonville ought to apply to these friends that come to the R.N.C., and we must always make that clear upfront,” he stated.

Florida has required guests from states with excessive an infection charges to quarantine for 2 weeks once they arrive. It’s unclear if that order can be lifted earlier than the conference.

The final time Florida hosted a Republican conference, in 2012 in Tampa, the celebration had to shorten it because of Tropical Storm Isaac, which later turned a hurricane. When Jacksonville hosted 2005 Super Bowl, it had to usher in cruise ships to complement the scarce variety of downtown resort rooms.

Mr. Trump is predicted to simply accept his celebration’s nomination on Aug. 27, 60 years to the day after certainly one of Jacksonville’s darkest days, when about 200 white supremacists led by the Ku Klux Klan brutally beat younger civil rights activists, most of them black, with baseball bats and ax handles. One of many demonstrators on what turned often called Ax Deal with Saturday was Rodney L. Hurst, who was 16 and president of the Jacksonville N.A.A.C.P. Youth Council.

“We’re going to have the commemoration of Ax Deal with Saturday this yr with or with out Donald Trump and the Republican Nationwide Conference coming to city,” stated Mr. Hurst, 76.

In a transfer extensively seen as politically expedient, two days earlier than the celebration introduced Jacksonville as its choose, Mr. Curry ordered {that a} outstanding bronze statue of an nameless Accomplice solder be taken down, a step the mayor had been beforehand unwilling to take.

Consultant Val B. Demings of Orlando, a contender to be Joseph R. Biden Jr.’s running mate, stated her mother and father tried to defend her and her siblings from racism once they have been rising up in Jacksonville, however they skilled it anyway. Whereas attending a newly built-in public college in sixth grade, Ms. Demings befriended a white lady.

“When she requested her mom about me coming over, her mom stated, ‘I’m not going to have that N at my home,’” Ms. Demings stated, referring to a racial slur.

The problem of confronting the virus and Jacksonville’s racist historical past whereas a nationwide political conference involves city has troubled town’s enterprise elite from the beginning, stated Nate Monroe, metro columnist for The Florida Occasions-Union newspaper, with the crush of recent infections solely intensifying these issues.

“Lenny Curry isn’t a person identified for altering his thoughts fairly often,” Mr. Monroe stated. “I don’t know what might probably derail the conference.”

Annie Karni contributed reporting from Washington. Kitty Bennett and Sheelagh McNeill contributed analysis.