Dr. Anthony Fauci is set to face another high-profile showdown with Sen. Rand Paul as the Kentucky Republican continues his yearslong investigation into the origins of COVID-19.

Paul has subpoenaed the former National Institute of Allergy and Infectious Diseases director to testify before a Senate committee Wednesday, renewing accusations that Fauci misled the public about the pandemic and the virus's origins. Fauci has repeatedly denied the allegations, calling them "preposterous."

Ahead of the hearing, Paul released more than 1,000 pages from Fauci's personal pandemic-era diary, arguing the entries show a gap between what Fauci privately believed and what he told the public.

Some of the material, including Fauci's early uncertainty about the virus as scientists raced to understand the outbreak, had already been described in his 2024 memoir and previous interviews.

Fauci's attorneys rejected Paul's allegations in a letter before the hearing, calling them "absolutely baseless" and part of an "obsessive and groundless vendetta."

The hearing marks Fauci's second appearance before Congress on COVID-19's origins since leaving government in 2022. During a career spanning more than five decades, Fauci advised seven presidents and led the nation's response to major public health crises, including HIV/AIDS, Ebola, anthrax and COVID-19.

More than 150 infectious disease experts and scientists signed a public letter defending Fauci, saying there is "no credible evidence" supporting the accusations against him and urging Congress to end what they called political "witch-hunts."

The hearing is expected to focus on whether National Institutes of Health-funded research in China contributed to the pandemic.

Most scientists say the available evidence still points to the virus emerging naturally from animals before spreading to humans in Wuhan, China.

While some researchers say a laboratory leak cannot be ruled out, there has been no conclusive evidence proving that theory. A Republican-led House subcommittee investigating the issue in 2024 also found no evidence that Fauci engaged in wrongdoing.

Paul is also expected to revisit allegations that Fauci misled Congress about National Institutes of Health funding for so-called gain-of-function research at a laboratory in Wuhan.

The NIH previously funded grants to EcoHealth Alliance, a New York-based nonprofit that collaborated with Chinese researchers studying bat coronaviruses.

Fauci has long maintained that the research did not involve the type of high-risk gain-of-function experiments that could have produced the virus responsible for COVID-19, saying it would have been "molecularly impossible" for those experiments to create the pandemic virus.

The federal government has since tightened oversight of certain gain-of-function research, including new restrictions announced after the pandemic.