Health Secretary Robert F. Kennedy Jr. heads to Capitol Hill on Wednesday on a potential collision course with the Republican who helped put him in the job: Louisiana Sen. Bill Cassidy.

It will be Kennedy’s first appearance in nearly a year before the Senate Health, Education, Labor and Pensions committee, which Cassidy chairs. The senator, who is up for reelection, cast the key vote to confirm Kennedy last year after securing a series of promises from Kennedy, including that he would preserve federal vaccine recommendations and regularly appear before the committee.

Kennedy has not kept those promises; Cassidy has limited his criticism of the health secretary to posts on social media and press statements. He has been vocal in his support of vaccines, including during the March confirmation hearing for Dr. Casey Means, a Kennedy ally who has questioned vaccines. Cassidy has not yet scheduled a vote to advance Means’ nomination.

Wednesday’s hearing will mark Kennedy’s first appearance before Cassidy since a confrontational Senate Finance Committee hearing in September and could offer the clearest sign yet of how the senator plans to handle those concerns. A spokesperson for Cassidy declined to comment on what the senator plans to ask Kennedy.

Kennedy is also expected to face questions from Sen. Thom Tillis, R-N.C., who is retiring next year and has said he plans to speak more freely about his views, including on members of Trump’s cabinet. (Tillis voted to confirm all members of Trump’s cabinet in 2025.)

Kennedy will appear before the Finance Committee in the morning and the HELP Committee in the afternoon.

In January, Kennedy overhauled the childhood vaccine schedule, reducing the number of recommended diseases for children to be vaccinated against from 18 to 11 — a move Cassidy later said in a post on X would “make America sicker.” The changes removed recommendations that all babies should be protected against hepatitis A, hepatitis B, RSV, dengue and two types of bacterial meningitis.

In March, a federal judge blocked those changes and put on hold the new members Kennedy appointed to the CDC’s vaccine advisory committee. The administration hasn’t yet appealed the ruling. But Kennedy signed off on new rules for the committee that could make it easier to work around the court’s decision.

Dorit Reiss, a vaccine policy expert at the University of California Law San Francisco, said that she hopes that Cassidy will hold Kennedy accountable.

“There’s a raging measles outbreak,” Reiss said. “Kennedy may have given lukewarm endorsements to the MMR vaccine but, as far as I know, hasn’t made any efforts to call on people to vaccinate or to do anything practical to reduce the risk.”

Kennedy has already testified at five congressional hearings over the last week. He has faced blistering criticism from Democrats over his vaccine policy and overhaul of federal health agencies. At one hearing, Kennedy said the U.S. has “done better” at preventing measles than any other country.

“Judging by Secretary Kennedy’s recent testimony to Congress, he is likely to continue to gaslight the Senate Finance and HELP committees,” said Lawrence Gostin, director of the O’Neill Institute for National and Global Health Law at Georgetown University. “He continues using terms like ‘world-class science,’ ‘rigorous evidence,’ and ‘radical transparency,’ when in fact he has done the opposite.”

In an emailed statement, Andrew Nixon, a spokesperson for the Department of Health and Human Services, called what Gostin said “a baseless accusation that doesn’t match reality.”

Another potential wildcard for Kennedy is Tillis, a Republican who is not seeking reelection, Reiss said.

During the September hearing, she noted, Tillis suggested that Kennedy had broken his promises on vaccines, saying, “I do also believe that some of your statements seem to contradict what you said in the prior hearing.”

“The fact that you’re a Republican doesn’t mean that you need to blindly accept [Kennedy’s actions],” Reiss said.

A spokesperson for Tillis did not immediately respond to a request for comment.

Kennedy could also face questions about his recent comments to overhaul the U.S. Preventive Services Task Force, a federal panel that makes recommendations on preventive services, including cancer screenings, as well as on President Donald Trump’s executive order meant to spur research into psychedelics.