RFK Jr. hearing live updates: Trump's pick to head HHS faces bipartisan skepticism

To win confirmation, Kennedy can only afford to lose three Republican votes -- assuming all Democrats oppose him.

So far, no Democrats have publicly said they'll be supporting his nomination.

Some Republicans to watch include Mitch McConnell, a polio survivor, and Bill Cassidy, who has said he believes Kennedy is "wrong" on vaccines.

Vice President JD Vance has been lobbying senators behind closed doors to support Kennedy, ABC News previously reported. A spokesperson for Vance did not respond to a request for comment.

According to financial disclosure documents, Kennedy's primary source of income in the past year was large sums of referral fees from multiple law firms, including one that is leading a civil lawsuit against Merck's Gardasil vaccine.

Democrat Elizabeth Warren, who sits on the finance committee that will be questioning RFK Jr., has criticized him for making "millions off of peddling dangerous anti-vaccine conspiracies."

Kennedy said he has resigned from his work with several law firms and that if confirmed he would no longer be involved in legal cases. But in a plan greenlit by federal ethics officials, Kennedy said he plans to retain his right to 10% of fees awarded in contingency cases with Wisner Baum, so long as the cases don't involve the U.S. government. The federal government is not a party in the civil lawsuit against Merck.

Read more about RFK Jr.'s involvement in the lawsuit here.

-ABC News' Anne Flaherty and Soorin Kim

White House press secretary Karoline Leavitt defended Robert F. Kennedy Jr. ahead of his confirmation hearing in an interview Wednesday on ABC News' "Good Morning America."

She said she rejected the criticism Caroline Kennedy aimed at her cousin in a recently published letter.

"I would reject those comments fiercely. RFK Jr is widely respected. He's a great man who has dedicated his life to public service and to research," Leavitt said. "He is pro-science, and he is committed to making this country healthy again."

"President Trump stands strongly behind him, and we look forward to watching him shine on Capitol Hill today in his confirmation hearing, just like all of the other nominees that President Trump has appointed to their respective agencies and departments," she added.

-ABC News' Molly Nagle

Caroline Kennedy, RFK Jr.'s cousin, wrote a letter to lawmakers warning she believed he is "unqualified" for the role.

She called him a "predator," shared disturbing details of his alleged behavior with animals and accused him of being "addicted to attention and power."

"The American health care system, for all its flaws, is the envy of the world. Its doctors and nurses, researchers, scientists, and caregivers are the most dedicated people I know. Every day, they give their lives to heal and save others. They deserve better than Bobby Kennedy – and so do the rest of us. I urge the Senate to reject his nomination," she concluded.

Read more about her letter to senators here.