Scientists with differing opinions about COVID-19 origins than Dr. Anthony Fauci keep it to themselves because because the top disease expert controls much of their funding, Sen. Rand Paul, R-Kentucky, told "Fox News Primetime" Tuesday night.

Paul subjected Fauci to stiff questioning during a Senate hearing chaired by Democrat Tina Smith of Minnesota earlier in the day, with Fauci quibbling over Paul's definition of gain-of-function research, which the lawmaker said came from a document sourced by another epidemiological expert. 

The senator went on to urge Fauci, the head of the National Institutes of Allergy & Infectious Disease, to reconsider prior testimony denying NIAID or NIH's particular funding endeavors at the Wuhan lab; where many believe COVID-19 originated – citing federal perjury laws.

Paul reacted further to Fauci's behavior and doubled down on his assertion that the 80-year-old Brooklyn, N.Y. native is not letting on as much as he should be on the topic of gain-of-function research funding.

He said he read aloud the NIH's definition of gain-of-function research, as well as a scholarly paper from a cellular biology expert working at Rutgers University in New Jersey, which described Wuhan lab official Dr. Shi Zhengli's work as the textbook definition of such experimentation.

"All Dr. Fauci could do was sputter and call me liar – but he never at any point in time addressed any of the facts that we laid out that the money he was giving to Wuhan was indeed for gain of function."

Paul chalked up the fact he is one of few prominent medical experts – as a doctor himself – speaking out and contradicting Fauci to the assertion that many others in medicine rely on NIAID funding – which Fauci has the ultimate say on.

The senator noted Fauci has worked at the Bethesda, Md.-based NIH since 1968. He was appointed the head of NIAID by President Reagan in 1984.

Host Brian Kilmeade went on to note that Johns Hopkins physician Dr. Marty Makary told him on the "Brian Kilmeade Show" earlier in the day that the NIH distributed $40 billion in research grants in 2020 – but less than one-half-of-one-percent went to COVID-related endeavors.

Makary said Fauci should be ashamed at that statistic, and Kilmeade asked Paul why there is not more outrage on the whole among the medical community:

"He has been [at NIH] for 40 years; probably 39 years too long. But he controls all the funding," Paul warned.

"So, people are deathly afraid of him. Researchers will not speak out. Why have there not been other scientists?"

Paul said that he receives letters from scientists that routinely contradict Fauci's prescriptions and public statements, but that they all offer the same regret: that they are afraid to speak out against what the health care bureaucrat says.

"They are very distrustful of what he is saying. They don't think he is making sense and reading the science accurately," said Paul.

"They're afraid to speak out because many of them are university scientists and they depend on NIH funds: To cross him means it's last money you will ever get."

Paul said Fauci has a "significant conflict of interest" in the Wuhan matter because he was "at the top of the food chain" of funding distribution and now claims none of it went to the dangerous gain-of-function research.

"All he is saying is oh, well, the research now doesn't meet our definition," he said. "He is dancing around the truth. Why? Because if this disease came from the lab and they were funding gain of function, guess what? There is at the very least moral culpability he has for the beginning of the pandemic."

Paul added that in 2012, Fauci made statements about the potential for a pandemic leaking from a lab, and that if such a situation occurred the research would be worth the risk.

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In the document Fauci prepared that year, he defended controversial gain-of-function research, saying that the "benefits" gained from the science "outweigh the risk" of an accidental pandemic breaking out.

"It is more likely that a pandemic would occur in nature, and the need to stay ahead of such a threat is a primary reason for performing an experiment that might appear to be risky," the paper read.

During the hearing Tuesday, Fauci responded to Paul's assertions saying he denies and "resents" them.

Fox News' Houston Keene contributed to this report.