Standing maskless in the White House Rose Garden on a sunny May afternoon, President Biden heralded some happy news. “If you’ve been fully vaccinated, you no longer need to wear a mask,” the president declared. “It’s vaxxed or masked.”

Less than three months later, amid rising cases driven by the delta variant and more breakthrough coronavirus infections, Biden was forced this week to back away from that proclamation. The administration issued new guidance Tuesday that encourages fully vaccinated Americans to wear masks indoors in places with substantial infection levels, encompassing more than 60 percent of the nation’s counties.

This new landscape — and, some say, the administration’s less than clear messaging — is complicating Biden’s efforts to show that he is still leading the United States out of the pandemic, his presidency’s central promise.

And this time, he must do it after Americans have enjoyed a taste of normalcy and been assured, including by the president, that they had turned a corner. This week’s developments mark the first significant retreat in public health guidance since Biden became president, and they set up a test of whether many Americans will agree to return to the tiresome protocols in place before shots became widely available.

To Biden’s supporters, the president is simply shifting his message, as he should, in response to an aggressively evolving virus. But the new masking guidelines also highlight questions that public health officials have been asking for weeks — whether the administration was too quick to relax guidelines in May and too fast to celebrate getting back to normal, especially with Biden inviting 1,000 guests to the White House for Independence Day.

“He certainly has a good story to tell about their efforts, and there’s probably a recognition that the resistance to vaccinations is very much behind this change,” said David Axelrod, who was President Barack Obama’s top political strategist. “But how far that goes as a shield against public dissatisfaction with where we are? I don’t know.”

Axelrod added, “Psychologically, it’s much harder to get people to charge up the hill once than to have them roll down the hill and have to charge up the hill a second time.” Americans, he said, will be “grumpy” about having to wear masks again.

The coming days could show whether more public frustration will be directed at Biden — who polls show is rating highly for his handling of the pandemic — or at the millions of Americans who are still refusing to get vaccinated.

Meanwhile, the latest guidance is notably complicated and may be difficult for many to follow. The Centers for Disease Control and Prevention is recommending the additional masking only in specific, frequently shifting areas — that is, counties with 50 or more infections per 100,000 people over a one-week period — and it applies only to indoor, publicly accessible spaces.

Beyond that, many Americans travel to and from different counties for home and work. The CDC is encouraging people to check its website for conditions in their area, but some experts said the new guidance could be most helpful to local health officials, who can issue rules appropriate for their area.

Republicans criticized the White House for what they described as frequent and rapid changes in direction.

Sen. Thom Tillis (R-N.C.) said he is “deeply concerned” that “the Biden administration’s contradictory decision will cause even more vaccine hesitancy, giving many Americans the false impression that the vaccines are not as effective as they were originally told.”

Others have raised questions about the relationship between the CDC, an academic-oriented agency charged with issuing scientific guidance, and the White House, which has the broader job of telling Americans the best course of action and encouraging them to follow it — a task that is not just scientific but also political, social and cultural.

The change this week has been swift and striking as the country faces more directly than ever the reality that the coronavirus has several more acts left. The administration’s shift had an immediate, stark visual impact: Because D.C. has a substantial infection rate, Vice President Harris on Tuesday donned a mask during an indoor discussion of Native American voting rights and asked her guests to do so.

The White House, eager to show it is abiding by the new guidelines, quickly sent an email directing all staffers to start wearing masks again. Congress’s physician recommended that anyone inside the Capitol complex should wear a “medical-grade filtration mask.”

Cities across the country, meanwhile, are reconsidering reinstituting mask mandates, and a growing number of organizations, including the federal government, are weighing vaccination requirements.

The White House and its supporters say their actions flow directly from the reality that scientists’ understanding of the coronavirus is constantly evolving — like the coronavirus itself. In May, the highly contagious delta variant was responsible for only a small percentage of infections; now it’s at more than 80 percent.

The administration’s new position, they say, is not a flip-flop but a prudent response to a virus that is evolving in aggressive and sometimes surprising ways. The administration’s willingness to adapt, they add, reflects integrity, not vacillation.

The White House this week attributed the new guidance to two factors: the increasingly evident danger of the delta variant and the ongoing refusal of many Americans to get vaccinated to help snuff out the pandemic.

“We are dealing with a much different strain of this virus than we were even earlier in the spring, back in May, when the masking guidance was done,” White House press secretary Jen Psaki said Tuesday.

Biden pointed his finger directly at those who are refusing to get immunized.

“We have a pandemic because of the unvaccinated, and they’re sowing enormous confusion,” the president said. “The more we learn about this virus, and the delta variation, the more we have to be worried and concerned. And there’s only one thing we know for sure: If those other 100 million people got vaccinated, we’d be in a very different world.”

Biden suggested he would seek to use the current setback to galvanize the country and continue driving up vaccination rates. And he promised the nations will not see the shutdowns and school closures of last year.

“Unlike 2020, we have both the scientific knowledge and the tools to prevent the spread of this disease,” Biden said in a statement after the release of the new CDC guidance. “We are not going back to that.”

Some of Biden’s allies and adversaries have said that the president fares better politically when covid-19 moves to the forefront of public awareness. His approval rating on handling the virus is frequently higher than his overall performance rating, they note, and the public appears to trust him far more than they trust Republicans on the issue.

Still, potential Republican challengers saw an opening to depict an overbearing bureaucracy whose edicts are heavy-handed and confusing.

Sen. Ted Cruz (R-Tex.) said the CDC has “lost its credibility,” adding that “it’s long past time we got back to trusting the American people, not unelected federal bureaucrats.”

Sen. Josh Hawley (R-Mo.) said the new guidance sends the wrong message “that no matter what you do, you will be required to wear a mask.”

Political adversaries are not the only critics of the administration’s latest moves. Many public health officials have argued since May that the Biden administration moved too swiftly to drop the masking recommendations for vaccinated Americans. Instead, they say, the administration should have relaxed mask rules only in areas with high vaccination rates and low community spread — essentially what the CDC is doing now.

Though the delta variant had not spread widely in the United States in the spring, it was ripping through other countries, including India.

“They shouldn’t have issued the guidance back in May,” said Leana Wen, a former Baltimore health commissioner. Once that guidance was issued, she said, the CDC “should have seen the extraordinary confusion that resulted at that time and backtracked then.”

Still, Biden faced considerable pressure at the time — from some public health experts as well as fellow Democrats — to give Americans a reason for optimism and hope and to show that the tough months of isolation, masking and vaccinations had yielded results. The administration’s celebratory tone in the spring unfolded against that backdrop.

White House officials bristled when asked about the politics of reversing course on masking.

“We are not, nor should we be, concerned about political implications of public health decisions,” said White House spokesman Chris Meagher. “We are concerned about the health and safety of the American people, which is why we are listening to the doctors at the CDC.”

But Biden allies acknowledged that the changing rules will require a difficult adjustment, particularly after a summer in which many children went mask-free. The new CDC guidance recommends universal mask-wearing in K-12 schools.

“Politically, are people going to be frustrated? Yes. Are people going to be upset? Yes,” said Randi Weingarten, who is the head of the American Federation of Teachers and is close with the Biden administration.

“But do we want to have a normal school year?” she added. “Do we want to have kids in school? Do we want to try to curb covid? Yes, yes and yes.”

To Democrats, the episode reflects the reality of combating a once-in-a-century global pandemic.

“Look, it would be much better for the White House if the virus went away,” said James Carville, who was a top strategist for President Bill Clinton. “We’re told, but we never believe it: This virus is in charge — we’re not.”