One anti-Fauci Twitter post last week said, “Sorry liberals but we don’t trust Dr. Anthony Fauci.”

The hashtag #FauciFraud has been used by more than 70 Twitter accounts, some posting hundreds of times a day, according to an analysis by The New York Times. Criticism of Dr. Fauci has also come from leading conservative voices and supporters of Mr. Trump, including Tom Fitton, the president of Judicial Watch, a conservative group; Bill Mitchell, the host of the far-right online talk show “YourVoice America”; and Shiva Ayyadurai, who has falsely claimed to be the inventor of email.

But Mr. Trump himself has praised the doctor. During Wednesday’s briefing, when a reporter asked about Dr. Fauci’s personal security, the president said: “He doesn’t need security. Everybody loves him.”

In fact, Dr. Fauci has earned plaudits from many medical experts and public health officials for his often grim assessment of the threats facing the United States from the coronavirus — even sometimes contradicting the president’s rosier outlook.

For weeks, Dr. Fauci’s remarks on television and at the White House stood in sharp contrast to the commentary from Mr. Trump’s fiercest supporters on Fox News, whose leading hosts repeatedly claimed that Democrats, the news media and public health experts were inflating the threat of the virus.

After the viral video of Dr. Fauci lowering his head, online attacks against him increased. A seven-year-old email that he wrote to an aide of former Secretary of State Hillary Clinton’s was posted online by The American Thinker, a conservative blog. In the email, Dr. Fauci praised Mrs. Clinton for her stamina during hearings into the 2012 attacks on the American compound in Benghazi, Libya. The blog suggested falsely that the email proved that Dr. Fauci was part of a secret group who opposed Mr. Trump.

Ironically, in the past several days, the president has largely adopted Dr. Fauci’s more dire warnings about the dangers of the rapidly spreading virus. On Tuesday, Mr. Trump called it a “great national trial unlike any we have ever faced before” and echoed Dr. Fauci’s language about the need to minimize its spread.

“It’s a matter of life and death, frankly,” Mr. Trump said, offering a sober assessment of the pandemic’s effect. “It’s a matter of life and death.”