The renewed sense of urgency was aimed at millions of people who have not yet been vaccinated and therefore are most likely to be infected.WASHINGTON — The director of the Centers for Disease Control and Prevention warned on Thursday that the United States was “not out of the woods yet” on the pandemic and was once again at a “pivotal point” as the highly infectious Delta variant ripped through unvaccinated communities.Just weeks after President Biden threw a Fourth of July party on the South Lawn of the White House to declare independence from the virus, the director, Dr. Rochelle P. Walensky, called the now dominant variant “one of the most infectious respiratory viruses” known to scientists.The renewed sense of urgency inside the administration was aimed at tens of millions of people who have not yet been vaccinated and therefore are most likely to be infected and become sick. Her grim message came at a time of growing anxiety and confusion, especially among parents of young children who are still not eligible to take the shot. And it underscored how quickly the pandemic’s latest surge had unsettled Americans who had begun to believe the worst was over, sending politicians and public health officials scrambling to recalibrate their responses.“This is like the moment in the horror movie when you think the horror is over and the credits are about to roll,” said Representative Jamie Raskin, Democrat of Maryland. “And it all starts back up again.”The choice by millions to reject the vaccine has had the consequences that public health officials predicted: The number of new cases in the country has shot up almost 250 percent since the beginning of the month, with an average of more than 41,000 infections being diagnosed each day during the past week — up from 12,000.The disease caused by the virus is claiming about 250 lives each day — far fewer than during the peaks last year, but still 42 percent higher than two weeks ago. More than 97 percent of those hospitalized are unvaccinated, Dr. Walensky said last week.The public health crisis is particularly acute in parts of the country where vaccination rates are the lowest. In Louisiana, Mississippi, Alabama and Florida, the number of daily new cases is up more than 200 percent in the past two weeks, driving new hospitalizations and deaths almost exclusively among the unvaccinated. Intensive care units are filled or filling in southern Missouri and northern Arkansas.The turnabout is forcing both political parties in Washington to grapple — so far in halting and tentative ways — with questions about what tone they should strike, what guidance they should provide and what changes they need to make to confront the latest iteration of the worst public health crisis in a century.The White House announced new grants on Thursday to local health offices for vaccines and increased testing in rural communities, even as administration officials said they were “making continued progress in our fight against the virus” and insisted that there was no need to reconsider their basic strategy. Although reports of so-called breakthrough infections in vaccinated people are growing, they remain relatively uncommon, and those that cause severe illness, hospitalization or death are especially so.But the surge in infections and hospitalizations in some parts of the country, even if limited mostly to people who have chosen not to be vaccinated, has presented Mr. Biden with an evolving challenge that could threaten the economic recovery and his own political standing.The stock market is wobbly. His administration is under new pressure to reimpose mask mandates, as Los Angeles County did this week. And the president’s top aides are on the defensive about their strategy to get the pandemic back in check.“It’s frustrating,” Mr. Biden acknowledged Wednesday night during a town hall event on CNN.The rise of the variant may also be changing the equation for some Republicans, who are seeing many of their own voters hospitalized — or worse. Representative Steve Scalise of Louisiana, the No. 2 House Republican, received his first shot on Sunday, noting “another spike” in the pandemic. The Fox News host Sean Hannity declared on his show, “I believe in the science of vaccinations.”On Capitol Hill on Thursday, House Republican leaders and elected doctors only grudgingly signaled their support for vaccinations, though even that support was mixed.Dr. Rochelle P. Walensky, the director of the Centers for Disease Control and Prevention, said that the United States was “not out of the woods yet” on the pandemic and was once again at a “pivotal point.” Stefani Reynolds for The New York Times“If you are at risk, you should be getting this vaccine,” said Representative Andy Harris of Maryland, a physician, adding, “We urge all Americans to talk to their doctors about the risks of Covid, talk to their doctors about the benefits of getting vaccinated and then come to a decision.”Representative Greg Murphy, Republican of North Carolina, said, “This vaccine is a medicine, and just like with any other medicines, there are side effects and this is a personal decision.”Their news conference was advertised as an attempt to “discuss the need for individuals to get vaccinated.” But it was dominated by efforts to promote an unproven theory that the Chinese released a virulent, human-made virus on the world and accusations that Democrats covered it up.The vaccines are working to keep those who have received shots out of serious danger, but charts tracking the pandemic that had been declining for months — heralded by Mr. Biden as proof that his approach was working — are heading sharply upward.The rapid sweep of the new variant has people questioning whether they need to retreat again from restaurants, movie theaters, bars, sporting events and their offices. What seemed like clear — and mostly positive — choices only days ago now seem muddy.White House officials deflected questions on Thursday about whether people who were vaccinated should begin wearing masks indoors again, as health officials in Los Angeles County ordered days ago. Jeffrey D. Zients, the coronavirus coordinator for the White House, said only that current C.D.C. guidance did not require it.“It’s up to each and every single American to do their own part,” he said. “We know everyone’s vaccination journey is different. We are ready to get more Americans vaccinated whenever, wherever they’re ready.”Amid the concern, one thing is clear: The variant has again upended hopes for an end to the pandemic and raised a new fear on the horizon — that a much-anticipated return to work and school could be disrupted after most of the country has spent nearly 18 months in stay-at-home seclusion.“I am worried about the fall,” said Representative Lauren Underwood, Democrat of Illinois and a registered nurse. “August is going to be rough. Back to school is going to be rough. We’re going to see more illness and more death.”Senator Roger Marshall of Kansas was among the Republicans signaling their refusal to go backward. “You don’t need to shut things down,” he said.  Stefani Reynolds for The New York TimesAndy Slavitt, a public health expert who recently left the Biden White House’s coronavirus response team, said the administration would not consider mandating vaccinations on the military or federal work force until the Food and Drug Administration gave permanent approval to the coronavirus vaccines, which are now under emergency use authorization.But, he said, final approval to the Pfizer vaccine is “within weeks to a short number of months.” Once that happens, he said, “everything should be on the table, and I can tell you that’s the attitude inside the White House.”Public school systems could also mandate vaccination at that point, just as they mandate vaccines for polio, measles, mumps and rubella — with some exceptions for religious or health reasons. That would quickly drive up vaccination rates.Beyond mandates, there are few obvious policy changes, since Congress has already showered the health authorities with funding for vaccination campaigns and made vaccines widely available. Representative Ami Bera, Democrat of California, who is a physician, suggested the Biden administration mount a public advertising campaign along the lines of smoking cessation campaigns that once featured a dying man smoking through his tracheotomy.“Let’s have an ad with a 20-year-old guy saying: ‘I didn’t take it seriously. I got it and I killed my grandmother,’” he said.President Biden now faces a more fragile public health reality that could threaten the country’s economic recovery and his own political legacy.Sarahbeth Maney/The New York TimesRepublicans have emphasized their refusal to go backward.“You don’t need to shut things down,” said Senator Roger Marshall of Kansas, a doctor. “Look, as far as I know, not one child under the age of 18 has died from Covid, unless they had some type of a serious health condition as well.”Deaths in American children are exceedingly low — 346 as of July 15 — but some of them most likely did not have underlying health conditions.So far, Republicans have also resisted raising alarm bells in conservative populations. The Kaiser Family Foundation reported at the end of June that 86 percent of Democrats had at least one shot, compared with 52 percent of Republicans.Policymakers feel hamstrung, in large part because once Americans resume life without masks and other restrictions, it will be difficult to go back. Vaccine and mask mandates would almost certainly prompt a fierce backlash, but they could also save lives.“We’ve all got this psychology, well it’s over, but intellectually we know it ain’t over,” said Representative Steny H. Hoyer of Maryland, the majority leader. He asked, “How do we get a society that had a tremendous sense of being locked up in a mask, then got free, to go back?”