The White House has accused leading infectious disease expert Anthony Fauci of playing politics days before the election in an interview about the coronavirus pandemic.
Dr Fauci told the Washington Post the US was in for a "whole lot of hurt".
He also offered an assessment of how both President Donald Trump and his Democratic rival, Joe Biden, are approaching the pandemic.
The US has recorded more deaths and cases than any other country.
According to data collated by Johns Hopkins University, deaths in the US have now passed 230,000, while more than nine million cases have been registered.
In his interview with the Post published on Saturday night, Dr Fauci warned that "all the stars are aligned in the wrong place as you go into the fall and winter season, with people congregating at home indoors."
"You could not possibly be positioned more poorly," he said.
When asked about the approaches of the two presidential candidates, Dr Fauci said Mr Biden was "taking it seriously from a public health perspective", while Mr Trump was "looking at it from a different perspective… the economy and reopening the country".
He said the US needed to make an "abrupt change" in public health practices and behaviours.
The comments drew a sharp rebuke from the White House, which accused Dr Fauci of attempting to bolster Mr Biden's bid for the presidency.
Spokesman Judd Deere said the comments were "unacceptable and breaking with all norms".
"As a member of the [US Coronavirus] Task Force, Dr Fauci has a duty to express concerns or push for a change in strategy, but he's not done that, instead choosing to criticise the president in the media and make his political leanings known by praising the president's opponent," he added in a statement.
Coronavirus has been a central issue in the run-up to Tuesday's presidential election.
Mr Biden has called the president's handling of the coronavirus pandemic an "insult" to its victims.
The Democratic candidate - who has not ruled out further lockdowns - pledged to "let science drive our decisions" if he is elected.
"Even if I win, it's going to take a lot of hard work to end this pandemic," he told voters this week. "I do promise this: We will start on day one doing the right things."
At a rally in Goodyear, Arizona, Mr Trump warned that a Biden presidency would lead to more lockdowns and economic misery for Americans.
"If you vote for Joe Biden it means no kids in school, no graduations, no weddings, no thanksgivings, no Christmas, and no Fourth of July together.
"Other than that you'll have a wonderful life. Can't see anybody, but that's alright," he said.
He cast the election as "a choice between a Trump super-recovery and a Biden depression".
Mr Biden has been observing Covid protocols at events ahead of the vote, while Mr Trump has been staging large campaign rallies without social distancing measures.
Polls suggest Mr Biden has a solid national lead over the Republican president, but his advantage is narrower in the handful of US states that could vote either way and ultimately decide who wins the White House.
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