WASHINGTON - President Joe Biden on Tuesday announced a summer-long effort to reach Americans still resistant to getting vaccinated, including going door-to-door and visiting places of worship, as he fights what growing evidence suggests is evermore entrenched resistance from vaccine holdouts.

In remarks from the White House, the president pointed to increased concerns over the delta variant of the coronavirus, which is more transmissible and has forced some European countries back into lockdowns, as he reiterated his exhortation that Americans get the vaccine.

The administration's renewed effort will center on getting shots into the hands of local doctors and medical experts, hoping that so-called trusted messengers can succeed in convincing people that the vaccinations are safe and beneficial where larger public messaging efforts have failed.

"Our fight against this virus is not over," Biden said. "Right now, as I speak to you, millions of Americans are still unvaccinated and unprotected. And because of that, their communities are at risk, their friends are at risk. The people that they care about are at risk."

Biden's tone was if anything more urgent than previously, as the country faces potential infection spikes in the fall. Standing in front of a backdrop that declared "We can do this," Biden sounded alternately like a salesman ("it's never been easier"), a supplicant ("please, please get vaccinated") and an Army recruiter ("it's a patriotic thing to do").

He stressed that the immunization drive is moving into a new phase, with mass vaccination sites closing down and an intensified focus on daily destinations, from offices to churches to festivals to pharmacies, so people can get shots when they are "just going in to get toothpaste or something else you need from a drugstore."

The president continued his recent pandemic balancing act between congratulating Americans on their success and warning against overconfidence. He said 2021 has been "a year of hard-fought progress" but that "we can't get complacent. . . . The best thing you can do to protect yourself and your family and the people you care about the most is get vaccinated."

The White House is increasingly confronting the reality of just how many Americans are refusing to roll up their sleeves despite a full-scale effort by the federal government. Tuesday's push comes two days after Biden fell short of a self-imposed goal that 70% of U.S. adults would have received at least one dose of vaccine by the Fourth of July.

Still, the number has hit 67%, and the White House held a large event on the South Lawn to celebrate "independence from covid-19."

More important than the national vaccination rate is the persistently low rate in some individual states as their residents resist Biden's entreaties, reflecting the fact that vaccine hesitancy is a product more of distrust than access.

All American adults ​became eligible for vaccinations on April 19, but daily vaccination numbers have fallen precipitously since they peaked in April. At one point, more than 3 million Americans received vaccines daily, but in recent weeks, that number has fluctuated, dipping as low as just over 700,00.

Nearly 3 in 10 Americans (29%) say they are not likely to get vaccinated, including 20% saying they will definitely not do so, according to a Washington Post-ABC News poll published last week. The 29% who say they are unlikely to get a vaccine is an increase over the 24% who said the same thing in April.

The decision about whether to get vaccinated increasingly splits along partisan lines. The survey finds that 86% of Democrats have received at least one shot, compared with 45% of Republicans. Another 7% of Democrats say they are likely to do so, compared with 4% of Republicans.

In a nod to such partisan fissures, Biden's comments were carried live by CNN and MSNBC on Tuesday afternoon, while Fox News opted not to cover them. And while many GOP governors and lawmakers strongly advocate vaccination, others frame the vaccine push as an infringement on Americans' freedom.

"People have a choice. They don't need your medical brown shirts showing up at their door ordering vaccinations," Rep. Marjorie Taylor Greene, R-Ga., wrote on Twitter, along with a video of Biden discussing the administration's door-to-door efforts to get people vaccinated. "You can't force people to be part of the human experiment."

The Brownshirts, also called the SA, were a violent paramilitary group linked with the Nazis who operated outside the law, attacked Jews and leftists and were critical to Hitler's rise to power.

The refusal of the unvaccinated has potential consequences for the country's return to normal. Unvaccinated people have a greater chance of getting infected with the virus, giving it an opportunity to mutate to a more transmissible form, or one that can sicken people who have been inoculated.

As the delta variant has spread, the World Health Organization has reiterated its recommendation that all people, including those who are fully vaccinated, wear masks to slow the spread of the virus. The Centers for Disease Control and Prevention has not revised its guidance for Americans that fully vaccinated people do not need to wear masks in most settings.

Biden on Tuesday warned about the dangers of remaining unvaccinated as the delta variant moves through the United States, particularly for younger people who have lagged in getting shots.

"It seems to me this should cause everybody to think twice and it should cause reconsideration, especially young people who may have thought that they didn't have to be vaccinated, didn't have to worry about it, didn't have to do anything about it up to now," Biden said. "But the good news is that our vaccinations are highly effective, fully vaccinated Americans have a high degree of protection, including against this delta variant."

He stressed that over the next few weeks, the administration's efforts would be increasingly granular, seeking to weave vaccinations into the patterns of daily life.

His administration will try to get the vaccines in the hands of pediatricians, hoping they will get children vaccinated before they return to school or as they get physicals in advance of playing fall sports. The government will expand the use of mobile clinics and try to create opportunities for more people to get vaccinated at their workplace.

"Now we need to go community by community, neighbor by neighbor and in some cases door to door to get help to the communities that need it," Biden said.

In some ways, the administration's latest efforts are simply an escalation of the vaccination campaigns officials have been operating for months, particularly the reliance on trusted local leaders. Biden on Tuesday acknowledged the limits of what the federal government can do, appealing to the collective action of the American public.

"The bottom line is, my administration is doing everything we can to lead a whole-of-government response at the federal, state and local levels to defeat the pandemic," he said. "But we need everyone to do their part."