Consumer group Public Citizen and healthcare coverage advocacy group Protect Our Care co-wrote a letter that they plan to send on Friday to all 100 Senators, urging them to announce their opposition to Kennedy. They recruited 85 other non-governmental groups to join the letter including the Union of Concerned Scientists, the NAACP and the National Organization for Women.

"If Robert F. Kennedy, Jr. takes command of the Department of Health and Human Services, we will face lies and disinformation at an unprecedented scale that are capable of unwinding a century of progress on fighting disease and promoting public health," the letter, seen by Reuters, said.

Republican President Donald Trump announced his choice of Kennedy in November for the position. He appears before the Senate to answer questions about his nomination on Wednesday.

Most of the opposition is driven by his stance on vaccines, but a group founded by former Republican Vice President Mike Pence also objects to Kennedy's comments in support of abortion rights.

A spokesperson for Kennedy and the White House did not respond to requests for comment.

With Republicans' slim three-seat majority in the Senate, the party wants to ensure unity behind Kennedy, and no Republican senator has so far voiced opposition.

Kennedy has long sown doubts about the safety and efficacy of vaccines that have helped curb disease and prevent deaths for decades. He disputes the anti-vaccine characterization and has said he would not prevent Americans from getting inoculations.

But he has led the anti-vaccine group Children's Health Defense and in a 2023 interview with podcaster Lex Fridman said no vaccines are safe and effective.

Kennedy has said he wants to work to end chronic disease, break any ties between employees at the U.S. drugs regulator and industry and advise U.S. water systems to remove fluoride.