At one point, Mr. Kennedy suggested that Black people should adhere to different immunization schedules. At another, a Republican senator suggested that American children are over-medicated and recalled the days when parents disciplined their children with a belt. Senator Maggie Hassan, Democrat of New Hampshire, cried while talking about her son. Mr. Kennedy yelled at Senator Bernie Sanders, Independent of Vermont, who yelled back.
In the middle of it all was Senator Bill Cassidy, who is a doctor and the committee chairman. A Louisiana Republican, Mr. Cassidy is on the fence about Mr. Kennedy. He opened the hearing by declaring that he was deeply troubled by Mr. Kennedy’s “past of undermining confidence in vaccines.” By the end, he wondered aloud whether Mr. Kennedy, 71, could change.
“Will you continue what you have been or will you overturn a new leaf?” Mr. Cassidy asked, adding that Mr. Kennedy, with his devoted fan base, could have an “incredible impact” were he to declare vaccines safe.
“That’s your power,” Mr. Cassidy said. “So what’s it going to be? Will it be using your credibility to support” vaccination, “or will it be using credibility to undermine? I’ve got to figure that out for my vote.”
Mr. Cassidy also serves on the Senate Finance Committee, which interviewed Mr. Kennedy on Wednesday and will determine whether Mr. Kennedy’s nomination receives consideration from the full body. The finance panel, which is likely to vote next week, has 14 Republicans and 12 Democrats. If Mr. Cassidy votes against Mr. Kennedy, causing the committee to deadlock, the Republican leader, Senator John Thune of South Dakota, could use a procedural tactic to force the nomination to the floor.
In a narrowly divided Senate, Mr. Kennedy can afford to lose only three Republican votes if he is to win final confirmation, provided all Democrats oppose him. Senator Lisa Murkowski, Republican of Alaska, also expressed misgivings about Mr. Kennedy’s views on vaccines on Thursday. And Senator Mitch McConnell, a Kentucky Republican who is a polio survivor, has not said how he will vote.
For the second straight day, Mr. Kennedy appeared to have little familiarity with the sprawling health programs he would oversee if confirmed to run the Health and Human Services Department. Pressed by Ms. Hassan, he was unable to correctly identify the different components of Medicare, the health insurance program for tens of millions of older Americans. On Wednesday, he had stumbled through a response to questions about Medicaid, the program for low-income people.
Mr. Kennedy on Thursday committed to a broad anti-abortion agenda, despite past comments supporting abortion rights. A former heroin user, Mr. Kennedy also promised to support the distribution of opioid addiction medicine to address a crisis still claiming roughly 100,000 lives every year.
But the bulk of the hearing was devoted to whether Mr. Kennedy was willing to accept consensus in scientific research. Mr. Kennedy frequently asserted that if lawmakers were to show him vaccine data that proved him wrong, he would apologize. Democrats fired back, telling Mr. Kennedy that the research he was asking to see had long been public.
Mr. Cassidy tested Mr. Kennedy’s promise in real time, reading the results of a broad study discrediting links between autism and vaccines.
“I’m a doc trying to understand. Convince me that you will become the public health advocate, but not just churn old information so that there’s never a conclusion,” he said.
Mr. Kennedy said that there were other studies he could show Mr. Cassidy.
Republicans on the panel mostly defended Mr. Kennedy’s approach, saying that scientific knowledge was volatile and rarely settled.
“We’re so consensus-driven that the science says this,” Senator Rand Paul of Kentucky, who is also a doctor, said, criticizing the government’s role in encouraging vaccination. “Well, science doesn’t say anything. Science is a dispute, and 10 years from now we could all be wrong.”
There were a number of times when emotions ran high.
Mr. Kennedy and Mr. Sanders shouted at one another after Mr. Kennedy, addressing Mr. Sanders as “Bernie,” accused the senator of taking money from pharmaceutical manufacturers. A furious Mr. Sanders acknowledged that $1.5 million of the $200 million he had received during his 2020 presidential race had come from drug companies. “I had more contributions from workers all over this country!” Mr. Sanders said angrily, adding, “Not a nickel from corporate PACS!”
At another point, Ms. Hassan broke down in tears while talking about her 36-year-old adult son, who has cerebral palsy. She said that when the first study about autism and vaccines came out — referring to research by a now-discredited doctor, Andrew Wakefield, that has been retracted and debunked — “it rocked my world.”
Her voice breaking, Ms. Hassan said not a day had gone by when she did not think about what she did when pregnant with her son that might have caused his condition. “So please do not suggest that anybody in this body of either political party doesn’t want to know what the cause of autism is,” she said.
But it is Mr. Cassidy, the soft-spoken physician from Louisiana, whose decision will ultimately be critical. He opened the hearing by recounting the story of a patient who needed a liver transplant; caring for her, he said, was the “worst day of my medical career” because he knew a vaccine could have prevented her fate.
“My phone blows up with people who really follow you, and there are many who trust you more than they trust their own physician,” Mr. Cassidy told Mr. Kennedy as the hearing began. “And so the question I need to have answered is, what will you do with that trust?”
The hearing also demonstrated the divisions within the Republican Party about vaccination. Three Republican senators — Mr. Paul, Markwayne Mullin of Oklahoma and Tommy Tuberville of Alabama — praised Mr. Kennedy for raising questions about vaccination.
When Mr. Paul suggested that infants did not need to be vaccinated against hepatitis B at birth, Mr. Cassidy stepped in to contradict him. Mr. Tuberville, a former football coach, announced at the hearing that his son and daughter-in-law had “done their research” and were not going to have their child vaccinated on the recommended schedule.
The hearing made clear that Democrats believe their strongest tactic is to hammer away at Mr. Kennedy’s views on vaccination. In response to a question from Senator Angela Alsobrooks, a Maryland Democrat who is one of two Black women on the panel, Mr. Kennedy defended his previous assertion that Black people should be on a different vaccine schedule than white people, saying there was research suggesting that Black people needed fewer antigens, the components of vaccines that provoke an immune response.
Ms. Alsobrooks cut him off. “Mr. Kennedy, with all due respect, that is so dangerous,” she said. “Your voice would be a voice that parents would listen to.”
Allies of Mr. Kennedy’s have expressed concern that the focus on vaccines could derail his confirmation; they would much rather he talk about issues that have bipartisan support, such as encouraging healthy eating and reversing the nation’s chronic disease epidemic. But Mr. Kennedy did not shy away from the vaccine debate.
His supporters were thrilled with his performance on Thursday. Zen Honeycutt, the founder of Moms Across America, an advocacy group that promotes healthy eating, said Mr. Kennedy’s exchange with Mr. Sanders exposed that “corruption in the system was not just in the system,” but within Congress, too.
Here’s what else to know:
- Disputed claims: The New York Times fact-checked Mr. Kennedy in real time. Read more here.
- Votes needed: The panel Mr. Kennedy appeared before on Wednesday, the Senate Finance Committee, is the one that will vote on whether to send Mr. Kennedy’s nomination to the full Senate for a final confirmation vote. Mr. Kennedy can lose only three Republican votes and still be confirmed if Democrats remain united in their opposition to him. Here’s a look at the handful of Republicans who hold his fate.
- More confirmation hearings: Mr. Kennedy isn’t the only one of President Trump’s more polarizing cabinet picks who appeared on Thursday on Capitol Hill. Kash Patel, Mr. Trump’s choice to lead the F.B.I., testified before the Senate Judiciary Committee, while Tulsi Gabbard, Mr. Trump’s pick to oversee the nation’s intelligence agencies, went before the Senate Select Committee on Intelligence. Read more about all the hearings here.