By Kevin Liptak and Jeremy Diamond, CNN

(CNN) -- Reluctance among conservatives to get vaccinated against Covid-19 has caused growing concern inside the White House, according to people familiar with the matter, even as President Joe Biden's administration rapidly scales up nationwide efforts to administer shots.

The topic has been the subject of several high-level conversations between administration advisers and health experts, including at the presidential level, the officials said.

Conservatives will now be one of the primary target audiences for a massive public relations campaign that could launch as early as next week, officials said. And the administration, through its partners, has been working with NASCAR, country music organizations and several rural organizations on vaccine confidence efforts meant for conservative eyes and ears.

Still, the one person whose public vaccine endorsement some experts say would be influential, President Donald Trump, appears unlikely to work in any capacity with the current administration to promote the shots. And polls show the number of Trump voters who say they will never get the vaccine is increasing.

The push to convince people who did not vote for Biden to get vaccinated underscores the difficulty in navigating the fraught politics of the pandemic. The vaccine joins masks, social distancing and lockdowns as a health measure that divides Americans by party, in large part fueled by conservative media distortions.

About 47% of people who supported Trump in the 2020 election said they would not get a Covid-19 vaccine if it became available to them, a recent NPR/PBS NewsHour/Marist poll found.

Trump, who received the vaccine in private before leaving the White House in January, did not participate in a PSA with other former presidents promoting the vaccine. But he encouraged his supporters to get it in an interview this week.

"I would recommend it, and I would recommend it to a lot of people that don't want to get it, and a lot of those people voted for me, frankly," he said during a phone interview on Fox Business on Tuesday. "It is a great vaccine. It is a safe vaccine and it is something that works."

Boosting confidence

The former President's comments caught the attention of White House officials, who had been working to devise ways to boost vaccine confidence in a population where they acknowledge they hold little sway. Among Biden's advisers, there has been an assumption that getting Trump involved in promoting the vaccine would inevitably require giving him some credit for helping develop the products quickly when he was president, something he has craved since leaving office.

A day before Trump's comments, Biden's press secretary suggested it shouldn't require an "engraved invitation" for him to promote the vaccine. And Biden himself downplayed the effect Trump could have on convincing his supporters to get vaccinated.

"The thing that has more impact than anything Trump would say to the MAGA folks is what the local doctor, what the local preachers, what the local people in the community say," Biden told reporters.

The President was reflecting, in part, the findings from a focus group conducted by Frank Luntz, a Republican strategist who convened 19 Trump voters over the weekend. The session was designed to test vaccine messaging strategies.

Inside the White House, officials took note of the focus group's findings, which included a show of hands on who would be more influential in whether the participants choose to get the vaccine. All of them said their own doctors would hold more sway than Trump.

"Trump's participation will help," Luntz told CNN on Friday. "It should have come four months ago, five months ago. It should have come back in November, December, January -- better late than never. But there are arguments that work even better than that."

"When it comes from the local doctor -- your doctor -- that's more believable than any politician, than any government agency, than Dr. Fauci, than anyone," he said.

Initially, White House efforts to combat vaccine hesitancy focused on minority populations, who have shown higher levels of such reluctance in some polls. But officials have come to realize in recent weeks that conservatives and Republicans will require similar efforts, leading to adjustments in strategy.

Bipartisan effort

The White House has been closely coordinating with the COVID Collaborative, a bipartisan association of policy experts and leaders, which has turned its attention to boosting public confidence in the coronavirus vaccines.

John Bridgeland, the group's co-founder and CEO, said White House officials began contacting his organization to discuss and align on the strategy and messaging around conservatives across the country as soon as a trove of data began pointing to conservatives as the most vaccine-hesitant constituency. Since then, Bridgeland has been in touch with the White House multiple times a week.

"They've been anxious about it because they know they can't get to herd immunity without it," said Bridgeland, who served as Domestic Policy Council director under President George W. Bush. "They recognize they're not perfectly positioned to be reaching conservatives and Republicans, and so we've been coordinating very closely."

Bridgeland said his group has helped the White House with a key part of its outreach to conservatives: arming trusted local leaders -- including doctors, religious leaders and others -- with the facts about the safety and efficacy of vaccines.

The COVID Collaborative will be announcing new partnerships with sports and faith-based groups in an attempt to reach conservatives and is also sending former New Jersey Gov. Chris Christie on a media tour across the country. The former Republican presidential candidate previously delivered a public mea culpa for not wearing a mask at a Trump Rose Garden ceremony before being hospitalized with Covid-19.

The ad campaign is set to debut in the coming weeks. It is expected to include spots on television, radio and the internet and to seek to have conservatives as one of its main focus, though people of color and younger Americans will also be targeted.

The precise timing of the campaign's launch is still being determined, and officials said they were highly attuned to not launching the campaign until there were sufficient doses available for people who want them.

What exactly those ads look like has been closely held, and one person says nondisclosure agreements were signed in the production of them. But officials said they wanted to focus beyond "celebrities getting shots" to home in more narrowly on trusted messengers like religious leaders and doctors.

There will still be some celebrities, officials said, though they will be only part of the larger strategy of using voices that hold meaning for people skeptical of the vaccine.

"I think all that stuff helps marginally," said Andy Slavitt, Biden's senior adviser for Covid-19 response, on CNN. "What people say, particularly conservatives, is that they don't want an authority to try and convince them to take the vaccine. What they want is actually not all that different from what other folks want, which is they've got a series of questions and they don't want to feel like they're being manipulated."

Other avenues

The administration is hopeful that another aspect of the vaccine push will prove more influential: sending money to local communities to produce their own materials using trusted voices in their towns and states.

The White House's coming vaccine campaign will include "sizable efforts" to "micro-target" communities of people who are experiencing vaccine hesitancy and access issues, one White House official said, emphasizing that the administration's efforts won't just center on people who are skeptical of the vaccines but also those who are reluctant to get vaccinated because they view it as an inconvenience and those who cannot easily access vaccination sites.

That is why the White House plans to pair its vaccine confidence efforts with the continued expansion of mobile vaccination clinics and the use of community health centers to target harder-to-reach populations, including rural Americans, many of whom voted for Trump. The administration also plans to send funding to organizations that can best reach certain audiences.

The White House has already deployed -- and has plans to continue using -- Dr. Francis Collins, the head of the National Institutes of Health, who also served in the Trump administration, to speak to conservative religious audiences. Collins is an evangelical Christian and made several appearances on the Christian Broadcasting Network and its program "The 700 Club."

Collins and Dr. Anthony Fauci, the leading infectious disease expert, who has assumed renewed prominent in the Biden administration, also met with evangelical leaders this week to discuss the vaccine efforts.

Ultimately, officials said they were most hopeful that once more Americans become vaccinated -- proving its safety -- that skeptics will drop their misgivings.

"The vaccines tell a great story," Slavitt said. "Just getting that information out to people is important."