WASHINGTON (AP) — Republican politicians are under increasing pressure to speak out to persuade COVID-19 vaccine skeptics to roll up their sleeves and take the shots as a new, more contagious variant sends caseloads soaring. But after months of ignoring — and, in some cases, stoking — misinformation about the virus, experts warn it may be too late to change the minds of many who are refusing.

In recent news conferences and statements, some prominent Republicans have been imploring their constituents to lay lingering doubts aside. In Washington, the so-called Doctors Caucus gathered at the Capitol for an event to combat vaccine hesitancy. And in Florida, Republican Gov. Ron DeSantis this week pointed to data showing the vast majority of hospitalized COVID-19 patients hadn't received shots.

“These vaccines are saving lives," said DeSantis, who recently began selling campaign merchandise mocking masks and medical experts.

The outreach comes as COVID-19 cases have nearly tripled in the U.S. over the last two weeks, driven by the explosion of the new delta variant, especially in pockets of the country where vaccination rates are low. Public health officials believe the variant is at least twice as contagious as the original version, but the shots appear to offer robust protection against serious illness for most people.

Indeed, nearly all COVID-19 deaths in the U.S. are now people who haven't been vaccinated. Nonetheless, just 56.2% of Americans have received at least one vaccine dose, according to the Centers for Disease Control and Prevention.

Overall, only 51% of Republicans said in mid-June that they had received at least one vaccine dose, versus 83% of Democrats, according to an AP-NORC poll. And many appeared to have made up their minds. Forty-six percent of those who had not been vaccinated said they definitely would not. Among Republicans, even more — 53% — said they definitely wouldn’t; just 12% said they were planning to.

“I think they’ve finally realized that if their people aren’t vaccinated, they’re going to get sick, and if their people aren’t vaccinated, they’re going to get blamed for COVID outbreaks in the future,” said GOP pollster Frank Luntz, who has been working with the Biden administration and public health experts to craft effective messaging to bring the vaccine hesitant off the fence.

But Luntz, who conducted another focus group Wednesday evening with vaccine holdouts, said there has been a discernible shift in recent weeks as skepticism has calcified into hardened refusal.

“The hesitation has transformed into opposition. And once you are opposed, it is very hard to change that position. And that’s what’s happening right now,” he said.

For months now, many conservative lawmakers and pundits have been actively stoking vaccine hesitancy, refusing to take the shots themselves or downplaying the severity of the virus. Republican governors have signed bills protecting the unvaccinated from having to disclose their status and tried to roll back mask mandates. And on social media, disinformation has run rampant, leading President Joe Biden to claim platforms like Facebook were “killing people” — a claim he later walked back.

At a recent conservative gathering, attendees cheered the news that the Biden administration was falling short of its vaccination goals. Invoking the nation's top infectious-disease expert, Dr. Anthony Fauci, Rep. Lauren Boebert, R-Colo., warned, the government: “Don't come knocking on my door with your Fauci Ouchie! You leave us the hell alone.”

Others, including former President Donald Trump, have repeatedly defended those who have chosen not to get vaccinated, stressing that the decision is a personal choice. Instead, they have pointed fingers at Democrats, suggesting they are to blame for the distrust.

“People are refusing to take the Vaccine because they don’t trust (Biden's) Administration, they don’t trust the Election results, and they certainly don’t trust the Fake News," Trump said in a recent statement.

But there were signs that messaging was changing this week, as conservative leaders advocated for the shots. On Fox News, host Sean Hannity implored his viewers to “please take COVID seriously,” saying, “Enough people have died." Iowa Sen. Chuck Grassley on Twitter encouraged “ALL eligible Iowans/Americans to get vaccinated.”

"The Delta variant scares me so I hope those that haven’t been vaccinated will reconsider,” he wrote.

Louisiana Rep. Steve Scalise, the House Republican whip, distributed pictures of himself receiving his first dose of the vaccine last weekend after months of holding out.

Senate Republican leader Mitch McConnell, a polio survivor who has consistently advocated on behalf of the COVID-19 shots, this week urged the unvaccinated to ignore “all these other voices that are giving demonstrably bad advice."

But the news conference convened by House GOP leaders on Thursday highlighted Republicans' competing messages on the virus.

Initially billed as an event where Republican doctors in Congress would address the rapidly spreading delta variant, the group instead spent most of its time railing against China and making unverified claims that the coronavirus came from a lab leak in Wuhan, a theory initially popular in far-right circles but now being seriously considered by scientists. They also attacked Democrats, including House Speaker Nancy Pelosi and the Biden administration, for not doing more to get to the bottom of the lab leak theory.

“The question is, Why are Democrats stonewalling our efforts to uncover the origins of the COVID virus?” said New York Rep. Elise Stefanik, the No. 3 Republican in the House.

Eric Ward, a senior fellow at the Southern Poverty Law Center who studies extremism, blamed vaccine reluctance on “nearly a year-and-a-half of right-wing rage machine rhetoric."

“Even conservative leaders now are having a hard time figuring out how to rein in what had primarily been a propaganda campaign, and they are now realizing their constituencies are particularly vulnerable," he said.

While some Republicans may be using strong words to promote the vaccine, few are proposing new measures to urge vaccination, such as incentives, public information campaigns or more aggressive outreach.

In New Hampshire, where shots have slowed to about 1,000 per week, Republican Gov. Chris Sununu said Thursday that there are no immediate plans to launch new initiatives.

“Right now, it’s folks’ individual responsibility. If someone hasn’t been vaccinated at this point, they’ve made that conscious decision not to,” he said. “The government’s job is to provide that open door. If you want the vaccine, here it is, nice and easy. If you need more information, here it is. So you have every tool in the toolbox available to you and your family to make that decision."

Other Republican continue to peddle falsehoods.

Rep. Marjorie Taylor Greene, R-Ga., was suspended from posting on Twitter for 12 hours earlier this week after spreading disinformation about vaccine-related deaths. And Charlie Kirk, the founder of Turning Point USA, a popular youth conservative advocacy group that last weekend hosted a conference that drew former Secretary of State Mike Pompeo and numerous members of Congress, suggested without evidence on his podcast that up to 1.2 million could have died after getting the COVID-19 vaccine.

In his focus groups, Luntz said that many skeptics have struggled to assess the veracity of the things they read and hear.

“There is so much misinformation out there, and they can't tell the difference between what is accurate and what is fake," he said. "So it makes it virtually impossible to communicate when they don’t know what to believe.”

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Associated Press writers Emily Swanson, Holly Ramer and Kevin Freking contributed to this report.