Washington DC - Health Secretary Robert F. Kennedy Jr. said Thursday that firing a top government scientist was "absolutely necessary," as he faced blistering criticism from Democrats urging him to resign over his steps to curb vaccines.

The Senate hearing, marked by sharp exchanges that often erupted into shouting matches, came days after the ouster of Sue Monarez, the former director of the Centers for Disease Control and Prevention (CDC).

Her dismissal, accompanied by several high-level resignations and hundreds of earlier layoffs, has plunged the nation's premier public health agency into turmoil.

In his opening remarks, Kennedy tore into the CDC's actions during the Covid pandemic, accusing the agency of failing "miserably" with "disastrous and nonsensical" policies, including masking guidance, social distancing, and school closures.

Marjorie Taylor Greene Marjorie Taylor Greene begs Trump to respond to request on behalf of Epstein abuse survivors

Donald Trump Trump brutally strips nearly half a million federal workers of their union rights

"We need bold, competent, and creative new leadership at CDC, people able and willing to chart a new course," he said, touting the health department's new focus on chronic disease and promoting prevention.

Monarez, the CDC director whom Kennedy previously endorsed, accused the secretary of a "deliberate effort to weaken America's public-health system and vaccine protections" in a Wall Street Journal op-ed Thursday.

Kennedy's explanation for her firing – as he told Senator Elizabeth Warren – was simply: "I asked her, 'Are you a trustworthy person?' And she said, 'No.'"

Once a respected environmental lawyer, Kennedy emerged in the mid-2000s as a leading anti-vaccine activist, spending two decades spreading voluminous misinformation before being tapped by President Donald Trump as health secretary in his second administration.

Since taking office, he has restricted Covid-19 shots to narrower groups, cut off federal research grants for the mRNA technology credited with saving millions of lives, and redirected funding toward research on debunked claims linking vaccines to autism.

Ron Wyden, the top Democrat on the Senate Finance Committee leading the hearing, set the tone by demanding Kennedy be sworn in under oath – accusing him of lying in prior written testimony when he pledged not to limit vaccine access.

Politicians MAGA congressman claims Ghislaine Maxwell "exonerated" Trump in DOJ testimony

Politicians Newsom trolls Trump with brutal photoshop meme in response to O'Donnell threat

"It is in the country's best interest that Robert Kennedy step down, and if he doesn't, Donald Trump should fire him before more people are hurt," Wyden thundered.

But Republican committee chairman Mike Crapo shot down the request, praising Kennedy's focus on chronic diseases such as obesity.

The exchanges only grew more ill-tempered.

Democratic Senator Maria Cantwell branded Kennedy a "charlatan" over his attacks on mRNA research, while Kennedy accused Senator Maggie Hassan of "crazy talk" and "making things up to scare people" when she said parents were already struggling to get Covid vaccines for their children.

Vaccines have become the flashpoint in an ever-deepening partisan battle.

Conservative-leaning Florida on Wednesday announced it would end all immunization requirements, including at schools, while a West Coast alliance of California, Washington, and Oregon announced they would make their own vaccine recommendation body to counter Kennedy's influence at the national level.

Republicans mostly closed ranks around Kennedy, though there was some notable dissent.

Senator Bill Cassidy, a physician whose support was key to Kennedy's confirmation, criticized his cancellation of mRNA grants. He was joined by fellow Republican doctor Senator John Barrasso and Senator Thom Tillis.

Cassidy pressed Kennedy on whether President Trump deserved a Nobel Prize for Operation Warp Speed, the public-private partnership that sped Covid vaccines to market.

Kennedy agreed Trump should have received the prize – but in nearly the same breath, praised hydroxychloroquine and ivermectin, drugs championed by conspiracy theorists that have been proven ineffective against Covid-19.