Robert F. Kennedy Jr., one of the nation’s most publicly recognized vaccine skeptics, is expected to face sharp questions Wednesday when he goes before a Senate panel for the first time as President Donald Trump’s pick to become health secretary.

A longtime environmental lawyer with no experience working in public health administration or medicine, Kennedy is known for his work in questioning the safety and efficacy of vaccines, including encouraging parents not to adhere to vaccination recommendations and helping to sue a vaccine manufacturer for what he alleged was marketing fraud.

If confirmed, Kennedy would be in charge of overseeing a vast federal agency that manages the health care of some 170 million Americans, regulates access to drugs and vaccines and tracks urgent outbreaks of diseases and foodborne illnesses.

According to two people familiar with the matter, Vice President JD Vance has been lobbying senators behind closed doors to support Kennedy. A spokesperson for Vance did not respond to a request for comment.

“Vance is working the phones to make sure Bobby gets over the line,” one person said.

In private meetings with senators, Kennedy has been telling senators that he’s not “anti-vaccine,” but rather that he wants more study, according to two people familiar with the discussions. That message appears to be working with several Republicans who say they are satisfied with his approach so far.

“I don’t agree with everything that he says but we certainly both think that there are opportunities in nutrition and to decrease the toxins that kids are exposed to,” said Sen. Roger Marshall, R-Kan.

Kennedy can only afford to lose three Republican votes on the Senate floor and still get confirmed. Sen. Mitch McConnell, a polio survivor, has panned Kennedy’s position on vaccines. Other Republicans have said they want Kennedy — a Democrat before aligning with Trump — to clarify his position on abortion rights.

So far, no Democrats have pledged to back him with Sen. Patty Murray, D-Wash., calling her private meeting with Kennedy “by far the strangest” meeting she’s had with a potential cabinet secretary.

“I was appalled at his answers and his lack of knowledge and what he was pushing on the American people,” she said. “We need somebody in charge of this agency that is going to make sure that our kids and grandkids … have the correct health care information not misinformation.”

Kennedy has been preparing for this week’s hearings by holding what insiders call “murder board sessions”– intense rounds of question-and-answer led by a group of advisors to prepare Kennedy for the most difficult questions he might face in the hearing rooms.

Lawyer Ken Nahagian, who escorted the nominee through the Senate hallways for meetings in recent weeks, led the sessions, according to two people with knowledge of his involvement.

Kennedy’s testimony comes as a scathing letter from his cousin and former U.S. ambassador, Caroline Kennedy, landed in the inboxes of senators calling him a “predator” who “preys on the desperation of parents of sick children” and is “addicted to attention and power.” Caroline Kennedy was the U.S. ambassador to Japan and later Australia during the Obama and Biden administrations.

“The American health care system, for all its flaws, is the envy of the world,” Caroline Kennedy wrote. “Its doctors and nurses, researchers, scientists, and caregivers are the most dedicated people I know … They deserve better than Bobby Kennedy – and so do the rest of us. I urge the Senate to reject his nomination.”

Outside groups critical of Kennedy said they were working to ensure senators understood the risks involved.

“RFK Jr. is so far out of the bounds of any kind of nomination for a post like this in my lifetime that there’s no question you could ask or no answer he could give that would make him qualified for this, or not dangerous in the post,” said Gregg Gonsalves, associate professor of epidemiology at Yale School of Public Health.

Gonsalves has been part of a letter-writing and phone campaign generating over 3,500 individual letters to senators from public health professionals, healthcare workers, community-based providers and scientists, asking the senators to reject Kennedy’s nomination.

Kennedy’s hearing comes at a time when much of the public has grown skeptical of the government, public health and the media, while gravitating toward online influencers.

The decline in public trust of health agencies in particular began during the COVID pandemic. Now, Americans are mostly split on saying whether they trust the Food and Drug Administration, according to a KFF poll released this week.

At the same time, 80% of Republicans say they trust Trump and Kennedy as they would their own doctors, the poll found.

ABC News’ Allie Pecorin and Mariam Khan contributed to this report.