National Institute of Allergies and Infectious Diseases (NIAID) director and top Biden medical advisor, Dr. Anthony Fauci, has clashed time and time again with Republican Senator Rand Paul of Kentucky.

But in a Tuesday Senate committee hearing, the two had their most contentious exchange yet, after Paul suggested Fauci lied to Congress regarding U.S. funding of gain-of-function research—in which scientists heighten the transmissibility of a virus to study potential emergence—at the Wuhan Institute of Virology in Wuhan, China, the virus's city of origin.

Paul and other Republicans have accused the National Institute of Health (NIH) of funding gain-of-function research that might have led to the virus, but the NIH said in a statement this past May:

"[T]he National Institute of Allergy and Infectious Diseases (NIAID) have for many years supported grants to learn more about viruses lurking in bats and other mammals that have the potential to spill over to humans and cause widespread disease. However, neither NIH nor NIAID have ever approved any grant that would have supported 'gain-of-function' research on coronaviruses that would have increased their transmissibility or lethality for humans."

The NIH has funded a grant for the U.S.-based EcoHealth Alliance for coronavirus in bats, which then conducted research with the Wuhan Institute of Virology, but the claim that the Institute conducted gain-of-function research to enhance the virus is, as of now, unsupported.

In comments to Paul during regarding the topic during a May committee hearing, Fauci said:

"Senator Paul, with all due respect, you are entirely and completely incorrect, that the NIH has not ever and does not now fund gain-of-function research in the Wuhan Institute."

Flash forward to Tuesday's hearing, where Fauci testified alongside other health officials.

Paul suggested that Fauci lied to Congress in his previous statements regarding gain-of-research funding, a claim that Fauci emphatically denied.

Watch the complete tense exchange below.

Paul cited a claim from Rutgers University professor Dr. Richard Ebright, which claimed that research conducted between EcoHealth Alliance and the Wuhan Institute on bat coronaviruses violated the 2014 moratorium on gain-of-function research—a claim heavily disputed by Broad Institute of MIT and Harvard molecular biologist Alina Chan.

Paul said:

"Dr. Fauci, knowing, that it is a crime to lie to Congress, do you wish to retract your statement of May 11 where you claimed that the NIH never funded gain-of-function research in Wuhan?"

Fauci was unflinching in his response:

"Senator Paul, I have never lied before the Congress, and I do not retract that statement. This paper that you are referring to was judged by qualified staff up and down the chain as not being gain-of-function. ... Senator Paul, you do not know what you are talking about, quite frankly, and I want to say that officially. You do not know what you are talking about."

Paul accused Fauci of "trying to obscure responsibility for four million people" dying globally from the virus.

Fauci said:

"I want everyone to understand that if you look at those viruses, and that's judged by qualified virologists and evolutionary biologists, those viruses are molecularly impossible to result in SARS-CoV-2."

Paul responded in an outburst:

"There were animal viruses that became more transmissible in human, and you funded it."

Fauci countered:

"You are implying that what we did was responsible for the deaths of individual, I totally resent that, and if anybody's lying here, Senator, it is you."

People trusted the assurances of Fauci, a career immunology specialist who's served under multiple Presidents of both parties, over the accusations from Paul, an eye doctor.

People praised Fauci's gloves-off response.

Meanwhile, others were similarly fed up with Senator Paul.