Dr. Anthony S. Fauci, the former government scientist both celebrated and despised for his work on Covid, on Monday forcefully denied Republican allegations that he sought to cover up the possibility that the pandemic originated in a laboratory, calling the accusation “absolutely false and simply preposterous.”

In a tense appearance before the House Select Subcommittee on the Coronavirus Pandemic, Dr. Fauci read out an email from February 2020 in which he encouraged a scientist worried about the possibility of a lab leak to report his concerns to the F.B.I.

“It is inconceivable that anyone who reads this email could conclude that I was trying to cover up the possibility of a laboratory leak,” Dr. Fauci testified.

Republicans on the panel have spent 15 months rooting through emails, Slack messages and research proposals for evidence against Dr. Fauci. In half a million pages of documents and more than 100 hours of closed-door testimony, the panel has so far found nothing linking the 83-year-old immunologist to the beginnings of the Covid outbreak in China.

But the panel has turned up emails suggesting that Dr. Fauci’s former aides were trying to evade public records laws at the medical research agency he ran for 38 years until his retirement in December 2022.

Some of those emails paint Dr. Fauci as being preoccupied with his public image; one April 2021 message from an aide said that while Dr. Fauci “prides himself on being like teflon,” he appeared to be “getting worried about the brown stuff hitting the fan” over questions about research funded by his agency, the National Institute of Allergy and Infectious Diseases.

Over the years, the agency gave research grants to EcoHealth Alliance, an American nonprofit group that partnered with international scientists — including some at a coronavirus lab in Wuhan, China, the city where the pandemic eventually started — as part of efforts to anticipate disease outbreaks.

Dr. Fauci’s appearance on Monday had all the trappings of a Washington media show. He entered the room through a back door, flanked by two burly security guards, and took his seat wearing a somber expression as a horde of cameraman pointed their lenses at his face.

In the audience sat one person wearing a “Fire Fauci” T-shirt. Others wore shirts saying, “Got Ivermectin?” — a reference to anti-parasitic medicine that Dr. Fauci’s critics have claimed, without evidence, worked against Covid-19.

Monday is lawmakers’ first chance to ask him about his agency’s record-keeping practices. For Republicans on the committee, the hearing is also the pinnacle, so far, of a long campaign against American scientists and health officials who they have suggested helped start the Covid pandemic.

No new evidence for the pandemic emerging from a lab, with or without the help of American taxpayer funding, has emerged in a series of high-profile hearings over the past year. In a report on Monday, titled “Republicans’ Fauci Flop,” Democratic lawmakers said that the panel’s Republicans had failed to advance the case that the coronavirus, known as SARS-CoV-2, came from a lab leak.

“Select Subcommittee Republicans have dedicated time and taxpayer dollars to a probe of federally funded research that has failed to meaningfully advance our understanding of the origins of SARS-CoV-2 and instead inflicted significant damage on Americans’ trust in our nation’s public health officials,” the report said.

But Dr. Fauci, who spent more than 50 years in government service and advised presidents of both parties on outbreaks of infectious diseases such as AIDS, Ebola, anthrax and the flu, was always the panel’s most prized quarry. In working under President Donald J. Trump and then President Biden, Dr. Fauci became the face of a Covid response that generated both veneration and frustration from Americans.

Appearing frequently on television, Dr. Fauci became a hero to Mr. Trump’s critics for correcting his falsehoods about the coronavirus. In the pandemic’s early days, he also downplayed the importance of masks for the general public, seeking to preserve them for medical workers, but then later encouraged mask use — prompting his critics to say that he was flip-flopping. And he publicly celebrated the Covid shots, turning the anti-vaccine movement against him.

The House subcommittee is headed by two doctors: Representative Brad Wenstrup, Republican of Ohio, is the chairman and Representative Raul Ruiz, Democrat of California, is the ranking member. Dr. Fauci faced a polite but chilly reception from Republicans, who have studiously been trying to build a case that lab work funded by the institute Dr. Fauci used to run may have contributed to the start of the Covid pandemic.

Republicans have focused in particular on funding the institute awarded to EcoHealth Alliance that was passed on to Chinese scientists. They have accused those scientists of cooking up the coronavirus in their Wuhan lab.

“Covid-19 wasn’t created by bats in a wet market,” Representative Marjorie Taylor Greene, Republican of Georgia, asserted last year as the subcommittee’s work got underway. “It was manufactured in a lab funded by Fauci. He tried to cover it up.”

Scientists and health officials have repeatedly noted that the coronaviruses being studied at the Wuhan lab with American funding — as well as other such viruses known to be the subject of research there — bore little resemblance to the one that set off the pandemic. A National Institutes of Health official testified last year before a different House committee that comparisons between the two were like “saying that a human is equivalent to a cow.”

In closed-door testimony before the House coronavirus panel in January, Dr. Fauci said, as he has previously, that it was possible that lab research had sparked the pandemic and that he kept “an open mind” about the origins. But, he said, “Some people spin off things from that that are kind of crazy.” And he reiterated that, in his view, the weight of evidence pointed toward the virus originating from animals before spilling into humans outside a lab.

In that testimony, Dr. Fauci referred to studies relying on early cases and viral genomes as well as sampling at an illegal wild animal market in Wuhan that suggested the pandemic-causing virus leaped from animals into people there.

“When I read the papers written by an international group of highly, highly respected evolutionary virologists, I lean much more heavily that this is a natural occurrence,” Dr. Fauci said.

Republican lawmakers seized on other parts of Dr. Fauci’s January testimony in advance of the hearing on Monday to attack the American Covid response. In a memo circulated on Friday, the Republicans highlighted comments from Dr. Fauci about, among other things, six-foot separation rules, masking policies and vaccine mandates.

Dr. Fauci is also likely to come under intense scrutiny over recent revelations that two of his former aides — Dr. David Morens, a senior adviser, and Greg Folkers, a chief of staff — sent emails during the pandemic in which they appeared to be skirting public records laws. In opening remarks posted online Sunday evening, Dr. Fauci said he “knew nothing” of Dr. Morens’s email practices, and said that Dr. Morens, who helped him write scientific papers, “was not an adviser to me on institute policy or other substantive issues.”

Some of the emails suggested that agency officials whose job it was to produce records under transparency laws helped colleagues circumvent those regulations, a possibility that a government accountability expert said he found “extremely concerning.”

The emails suggested that agency officials were worried not about the emergence of evidence related to the origins of the pandemic, but rather about the disclosure of notes in which they bluntly discussed “political attacks” on their research.

Still, Dr. Morens suggested in the emails that Dr. Fauci, too, was careful to avoid putting sensitive comments in places where journalists or members of the public might eventually be able to find them.

“I can either send stuff to Tony on his private gmail, or hand it to him at work or at his house,” Dr. Morens wrote of Dr. Fauci in the course of reassuring several scientists in April 2021 that they need not worry about public records requests. “He is too smart to let colleagues send him stuff that could cause trouble.”

Dr. Fauci disputed this in his opening remarks, writing that “to the best of my knowledge I have never conducted official business via my personal email.”