Joe Biden looked out at an audience of government scientists last week and recognized a mask-wearing Anthony Fauci, his top adviser on the coronavirus. “I’ve seen more of Dr Fauci than my wife,” he joked. “Who’s president? Fauci!”

The US president was visiting the frontline of the Covid-19 struggle, the National Institutes of Health in Bethesda, Maryland, where he unveiled a winter plan that includes a drive for vaccine boosters, free at-home testing and fresh requirements for international travelers.

But even as Biden preached to the converted on Thursday, he faced a new political threat. The Omicron variant was spreading rapidly from state to state, trailing uncertainty in its wake. “We’re going to fight this variant with science and speed, not chaos and confusion,” he promised, “just like we beat back Covid-19 in the spring and more powerful Delta variant in the summer and fall.”

Yet the Delta variant itself is far from beaten, underlining the perils of what may prove the defining issue of Biden’s presidency and the measure of its success or failure. He came into office promising to crush the coronavirus but, after at least one false dawn, that goal remains frustratingly elusive – and now Omicron could deliver another hammer blow.

Indeed, Biden’s aura of competence took a hit over the summer, partly because of the chaotic withdrawal from Afghanistan, partly because the Delta variant appeared to catch him by surprise. Its persistence has made a mockery of his July declaration that Americans could soon declare independence from the virus.

Laurie Garrett, an award-winning science writer, said: “I don’t think that anybody in the spring in the United States was operating with the correct level of alarm about the Delta variant.

“I would forgive many leaders for having an inability to read the situation adequately and recognise how dangerous it was but, once it was clear that the Delta variant was far more contagious, everybody should have gone into high gear and I do think there was a slowness in response.”

Biden’s swift travel bans on southern African countries in response to Omicron suggested a resolve to learn lessons from Delta; to some it looked like overcorrection. But the challenge this time is compounded by new extremes in the Republican party and rightwing media’s politicization of the pandemic.

On Thursday, the president acknowledged: “It’s become a political issue, which is a sad, sad commentary. It shouldn’t be, but it has been.”

His stated hope that the nation could now come together around his new plan will have struck some as optimistic to the point of naivety. Democrats accuse Biden’s opponents of weaponizing the virus and its variants against him with the long-term objective of denying him a second term.

Eric Schultz, a communications strategist who worked in the Obama administration, told the Associated Press: “It’s clear that Republicans have decided that the fate of the Biden presidency is tied to Covid. And Republicans have chosen to be on the side of the virus.”

Some Republicans have all but entrenched an anti-vaccination culture. Senators this week briefly threatened a government shutdown over mandates. Florida, Iowa, Kansas and Tennessee have extended benefits to workers who are fired or resigned over their employers’ vaccine requirements.

Leslie Dach, chair ofProtect Our Care, a healthcare pressure group, said: “They’re literally sacrificing the lives of their own voters on the altar of their personal politics. That’s just incontrovertibly true when you know that the death rate is 15 times higher and you see who is choosing not to be vaccinated in America. They’re basically meting out a death sentence for people.”

It seems to be getting worse. A day after news broke about the Omicron variant, Ronny Jackson, a Republican congressman from Texas and former doctor to Donald Trump, floated a groundless conspiracy theory. He tweeted: “Here comes the MEV – the Midterm Election Variant. They NEED a reason to push unsolicited nationwide mail-in ballots. Democrats will do anything to CHEAT during an election – but we’re not going to let them!”

Meanwhile, Lara Logan, a Fox News anchor, compared Fauci to the Nazi doctor Josef Mengele, known as the Angel of Death for the experiments he carried out on prisoners at the Auschwitz death camp. Michael Bornstein, a survivor of Auschwitz, described the comments as “disgusting”.

But Logan was not sanctioned by Fox News and, with Holocaust comparisons proliferating on rightwing social media, including even in merchandise, there are fears that America’s hyperpartisan atmosphere may have passed a point of no return, paralysing its Omicron response.

Garrett, author of The Coming Plague: Newly Emerging Diseases in a World Out of Balance, warned: “There is no possibility of working together. If you were going to write a scenario that was perfect for a virus to spread rampantly, having the humans at the edge of civil war every five minutes would be a perfect scenario.”

Despite these forces, the White House points to dramatic progress over the past year. Last Christmas less than 1% of adults were fully vaccinated; this Christmas that share will be 72%, including more than 86% of elderly people. More than 20 million children have been vaccinated – though under-fives still await approval – and 99% of schools are open.

But the pandemic has proved a tenacious foe with renewed surges in Michigan and other midwestern states threatening to overwhelm hospitals. About 40 million adults refuse to get vaccinated. Take-up of boosters – more essential than ever due to Omicron – has been sluggish: more than 100 million eligible people have not yet received the shot. Masks, empty offices and unpredictability persist.

The conflicting picture has left the president to juggle duelling messages, one encouraging a return to life as normal, the other urging continued precautions. There seems little prospect of a definitive ending or declaration of victory. Roughly 47% of Americans approve of Biden’s handling of the pandemic while 49% disapprove, according to a recent Washington Post/ABC News poll.

Michael Steele, former chairman of the RNC, said he would award the president about five marks out of 10 so far. “Given the success they had early on in getting the vaccine programme put in place, shots in arms and all that, when the [Delta] variant hit it caught them flat-footed and took them by surprise.

“The administration lost a lot of the gains it had made coming in the door because it shattered people’s confidence in their ability to not only handle what was going on but to actually know what was going on.”

Steele, a longtime critic of Trump, noted that calculated attacks and obstruction from the right present a further obstacle to the nation’s recovery from the pandemic. “Biden doesn’t want to further politicize Covid and yet you have Republicans and that’s all they know how to do.”

The situation, he added, is reminiscent of Barack Obama’s signature healthcare law, the Affordable Care Act, which Republicans spent years trying to repeal without offering a replacement.

Bill Galston, a senior fellow at former policy adviser to Bill Clinton, awards Biden a more generous eight of out of 10 for his coronavirus approach to date. “The reason that’s two clicks short of 10 is that I think the White House really went astray in early July when it did everything but hang a mission accomplished banner over the subject.

“As I recall, the president announced a summer of freedom. One of the things they have surely learned is that they’re at the mercy of events that they can neither foresee nor control in advance and so creating hopes that are then extinguished by events is really counterproductive.”

More than 780,000 Americans have now died from Covid-19. This week, at a White House press briefing, the Fox News reporter Peter Doocy asked: “Whatever happened to President Biden’s promise to shut down the virus?”

The press secretary, Jen Psaki, replied: “We’re working on it.”