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At the same time, Cassidy is fighting for his political future in next month’s primary, where President Donald Trump has in an unusual attempt to oust a sitting senator from his own party.

How Cassidy handles the hearings could affect his chances at a pivotal moment of his reelection campaign and set the tone for how Congress oversees the nation's health agenda at a time of rampant distrust and misinformation.

Cassidy hasn't faced Kennedy in public since September. In the subsequent months, Kennedy has attempted a that, if not blocked by , could undermine protections against diseases like flu, hepatitis B and RSV.

After a backlash, Kennedy has also pivoted to spending more time talking about less controversial topics like healthy eating — albeit with his own spin, including sharing exaggerated claims that various ailments can be .

Cassidy will have to decide on Wednesday whether to grill Kennedy on vaccines, an issue deeply important to him, or put their differences aside and prioritize loyalty to the Trump administration.

“He’s taken a risk showing any sort of resistance to RFK,” said Claire Leavitt, an assistant professor at Smith College who studies congressional oversight. “He may pay an electoral price for that.”

Cassidy has long advocated for vaccines

Cassidy has spent years walking a political tightrope. He's one of the few Republican senators who voted to convict Trump during an impeachment trial after the Jan. 6, 2021 attack on the U.S. Capitol.

As a liver doctor, he advocated for babies to receive hepatitis B vaccines shortly after birth, a step that could have prevented the disease in his patients. But when Trump nominated Kennedy, a longtime anti-vaccine activist, Cassidy supported him. He did so after securing various commitments, including that Kennedy would work within the current vaccine approval and safety monitoring system and support the childhood vaccine schedule.

The vote for Kennedy did not appear to mollify Trump. The president endorsed U.S. Rep. Julia Letlow, one of Cassidy's two primary opponents.

Cassidy also faces opposition from Kennedy's allies in the “Make America Healthy Again” movement, a group that includes both anti-vaccine activists and a wide variety of other crusaders for health and the environment. The MAHA PAC, aligned with Kennedy, has pledged $1 million to Letlow’s campaign. While the organization hasn't publicly said so, some have questioned whether the support is partly in retaliation against Cassidy for criticizing Kennedy's vaccine policy agenda.

“I’m not really sure what MAHA’s beef is," Cassidy told reporters earlier this month. “Let me point out that I am the reason that Robert F. Kennedy is now the secretary of HHS. He would not have gotten there otherwise."

Cassidy argues that he has “strongly supported” the MAHA agenda, especially when it comes to the fight against ultraprocessed foods. However, the physician-turned-senator acknowledged that he and MAHA have “disagreed on vaccines."

“We’ve seen, frankly, that I am right," Cassidy added, pointing to recent of children who were not vaccinated.

At a , he slammed Kennedy’s decision to slash funding for mRNA vaccine development. He interrogated Kennedy over his attempt to replace members of a vaccine committee, suggesting the new members could have conflicts of interest. He also raised concerns that Kennedy's vaccine policy decisions could be making it harder for Americans to get COVID-19 shots.

Later that month, Cassidy featuring former U.S. Centers for Disease Control and Prevention Director Susan Monarez, who was ousted by Kennedy less than a month into her tenure after they clashed over vaccine policy, and former CDC Chief Medical Officer Debra Houry, who resigned in August citing an erosion of science at the agency.

“I want to work with the president to fulfill his campaign promise to reform the CDC and Make America Healthy Again. The president says radical transparency is the way to do that,” Cassidy said at the time.

Experts say Cassidy's vaccine stance might not hurt him

Political consultants said they expect Cassidy’s primary opponents, Letlow and Louisiana Treasurer John Fleming, to seize on any sound bites from Wednesday’s hearings that can make Cassidy seem at odds with the Trump administration.

But Dorit Reiss, a vaccine law expert at UC Law San Francisco, said the political risk of advocating for vaccines may not be as strong among Republicans as some people assume.

“He’s probably not alienating voters by focusing on the issue and calling it out,” she said.

Louisiana political consultant Mary-Patricia Wray said she thinks most diehard MAHA voters already know who they are voting for, and it’s probably not Cassidy.

Instead, she said, he may still be able to appeal to Democrats who switch their party registration to vote in the primary, as well as a wide swath of still-undecided Republican voters who care about the same health care affordability issues he advocates for every day in Congress.

“If I was advising Bill Cassidy, I would tell him your goal here is not to get out unscathed,” Wray said. “Your goal is to prove that your consistency on issues regarding public health is an asset in your campaign, not a detriment.”

Election outcome will shape future oversight of HHS

Also at stake if Cassidy doesn’t make it to November’s general election is what will happen to his responsibility to oversee the massive U.S. Department of Health and Human Services as the chair of the Senate Health, Education, Labor and Pensions committee.

Leavitt, the Smith College professor, said seniority typically plays the most important role in who chairs Senate committees. She said another Republican in today’s increasingly hyper-partisan Congress may not be as willing as Cassidy to check Kennedy’s power.

Reiss, the vaccine law expert, said she wishes Cassidy had done more hearings or introduced legislation to rein in Kennedy. And she said the senator bears the blame for allowing Kennedy to bring unfounded vaccine fears into the government in the first place.

“His original sin, of course, was voting for Kennedy at all,” Reiss said.

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Associated Press writer Sara Cline contributed to this report.