Donald Trump Jr., the son of former President Donald Trump, has called infectious-diseases expert Dr. Anthony Fauci a "tyrant" for supporting vaccine mandates on U.S. flights.

On Monday, Fauci—director of the National Institute of Allergy and Infectious Diseases (NIAID) and the chief science adviser for President Joe Biden—said he supported the possible creation of a federal mandate that would require domestic air travelers to be vaccinated against COVID-19.

"When you make vaccination a requirement, that's another incentive to get more people vaccinated," Fauci said during a Monday morning appearance on the MSNBC news program Morning Joe.

He added that increasing vaccinations seems especially important now amid a new spike in COVID-related hospitalizations. "Anything that could get people more vaccinated would be welcome," Fauci said.

During the interview, Fauci didn't say whether he had actually suggested the idea to Biden.

Nevertheless, on Twitter, Trump Jr. criticized Fauci.

Trump Jr. wrote: "'Anything that could help more people get vaccinated would be welcome.' So what then, force people at gunpoint? Don't allow them to work or get food for their families? This man is a tyrant and a threat to whatever freedom we have left."

In a Monday evening CNN interview, Fauci explained, "It's on the table, and we consider it, but that doesn't mean it's going to happen."

Fauci doesn't independently set national policies on COVID-19. He is part of a team that makes suggestions for the president's administration to consider.

Trump Jr Fauci tyrant vaccine mandates flights
Donald Trump Jr. called Dr. Anthony Fauci a "tyrant" for supporting vaccine mandates on U.S. flights. Above, Trump speaks on stage during the Conservative Political Action Conference 2020 (CPAC) on Feb. 28, 2020, in National Harbor, Maryland.Samuel Corum/Getty

Trump Jr. has long criticized Fauci for recommending COVID-19 reduction measures that some conservatives see as unnecessary, power-hungry and infringing upon personal freedoms.

In July, Trump Jr. said that Fauci is "better at pitching baseballs than he is at science." He said that Fauci is merely a "bureaucrat" and said members of the press who suggest otherwise are being deceptive.

Trump Jr. made his remarks during the "America UnCancelled" event hosted by the Conservative Political Action Conference (CPAC).

In mid-June, Trump Jr. said of Fauci, "This guy's not a scientist, he's a bureaucrat in a lab coat—nothing more, nothing less. He's incompetent. He was in government because he probably couldn't have made it in the private sector."

Trump Jr. made the comments during an interview on the podcast The Truth with Lisa Boothe.

"[Fauci] wasn't good enough or smart enough, in my opinion, to do these things. But we gave him unfettered credibility, not based on anything, but based on the fact that he loved TV, he loved cameras, never met a camera he didn't like, and he'd give them everything they wanted each and every time," Trump Jr. said on the podcast. "I think he cost lives, and the media certainly is complicit in helping him do that."

In early June, critics accused Trump Jr. of issuing violent threats against Fauci after he shared a meme about Fauci's hypothetical murder. The meme suggested that Fauci's eventual death might be misrepresented as a suicide.

The meme raised concern especially since Fauci's high profile during the pandemic caused him to attract violent threats. Fauci has required additional security for his personal safety since the pandemic began.

Republicans and right-wing media figures have called for Fauci to be questioned or fired over his emails. Other Republican legislators and media figures have demanded Fauci's firing because the NIAID funded research at the Wuhan Institute of Virology (WIV). The WIV has been accused of causing the pandemic through an accidental leak.