Dr. Anthony Fauci says he would not serve under Donald Trump again should the former president win back the White House in 2024.

Trump had tapped Fauci, the director at the National Institute of Allergy and Infectious Diseases, to join the White House coronavirus task force at the beginning of the pandemic in 2020. So Fauci, now the chief medical advisor to President Joe Biden, was asked by CNN on Sunday whether he would work with Trump for a second presidential term if COVID-19 was still a threat, or if another public health crisis emerged.

Fauci responded, “Well, no,” with a small laugh.

Fauci was also asked whether he would have confidence in Trump’s ability to handle a pandemic again.

“If you look at the history of what the response was during the [Trump] administration, I think, you know, at best, you can say it wasn’t optimal,” Fauci responded. “And I think just, history will speak for itself about that.”

While the White House daily coronavirus briefings featuring Fauci and Dr. Deborah Birx made them both household names two years ago, the relationship between Trump and Fauci grew more strained as the president contradicted the advice of Fauci and the Centers for Disease Control and Prevention (CDC) for dealing with the COVID-19 pandemic. Trump, who tested positive for COVID-19 in October 2020, opposed many CDC mask wearing guidelines, for example, as well as social distancing measures. A complete timeline of Trump’s shifting stance on COVID and mask-wearing was detailed by MarketWatch’s Quentin Fottrell in 2020.

Since then, Fauci has become a frequent target of Trump’s attacks during the president’s rallies and on social media, with the former president repeatedly calling Fauci a “disaster.”

Fauci said that reporting to President Biden felt “liberating” after an “uncomfortable” year reporting to Trump.

“I can tell you, I take no pleasure at all being in a situation of contradicting the president,” he said shortly after Biden took office in January 2021. “You didn’t feel that you could actually say something, and there wouldn’t be any repercussion about it.”

See also: U.S. COVID cases at highest levels since November, while Northeast and Midwest are above delta peak

Trump has not officially declared that he is running for president in 2024, but he has been hosting rallies across the U.S. since losing the 2020 election.

At the Conservative Political Action Conference (CPAC) in February 2022, Trump falsely blamed his 2020 election loss on widespread voter fraud, claims that have been repeatedly debunked, and hinted that he may run for office again.

“We did it twice, and we’ll do it again,” Trump said of running in 2024.