Republican Sen. Bill Cassidy once hoped to forge an alliance with Robert F. Kennedy Jr. that could revitalize Americans’ trust in public health.
But just seven months after Cassidy’s vote paved the way for Kennedy to lead the US Health and Human Services Department, their relationship has reached a breaking point.
Cassidy led a hearing Wednesday with fired US Centers for Disease Control and Prevention chief Susan Monarez, granting her a high-profile platform to detail the internal clashes over vaccines that prompted Kennedy to oust her just four weeks after endorsing her appointment.
The hearing represented an extraordinary departure from congressional Republicans’ typical Trump-era aim of shielding the administration from criticism — instead elevating testimony that threatens to damage Kennedy’s credibility and amplify scrutiny of his efforts to reevaluate access to certain longstanding vaccines.
And it marked what allies of both Cassidy and Kennedy characterized as the lowest point of an already-tumultuous partnership, driven by the Louisiana senator’s growing alarm over Kennedy’s agenda and his increasingly apparent inability to rein him in.
“There is nothing worse if you’re Kennedy than to have your opponent get her own hearing,” said one person who has spoken recently with both men. “I think it’s a stunning decision.”
Cassidy refused to say if he still has confidence in Kennedy after the ousted CDC director appeared before the committee, telling reporters he wants to reserve judgment until the secretary has testified before the panel.
But the senator also told reporters that parents should not have confidence in the vaccine advisory panel made up of Kennedy appointees if they change the government’s longstanding recommendations.
“I think he needs the opportunity to speak to this, and I plan to hold any judgment until we have the opportunity to hear Secretary Kennedy,” he told reporters. “I will compliment him; he has echoed President Trump’s call for radical transparency and so I’m confident that he’ll come and confident that he’ll share his perspective.”
Cassidy wouldn’t say if he’d vote to confirm Kennedy again if the vote was held today. “I am steadfastly not judging until we have a chance to hear the secretary’s perspective,” he replied.
The session in front of the Senate’s health committee comes at an especially delicate moment for both Kennedy and Cassidy, who each face mounting political pressures.
The abrupt firing of Monarez sparked backlash from the public health community and lawmakers from both parties, putting Kennedy on the defensive over allegations he pressured Monarez to rubber stamp an anticipated revamp of the nation’s vaccine policies regardless of the underlying scientific evidence.
Though President Donald Trump remained publicly supportive of Kennedy following a fiery hearing earlier this month where the HHS secretary branded Monarez a liar, the episode has drawn the ire of a growing contingent of GOP senators.
Several Republicans, including close Trump allies Sens. John Thune of South Dakota and John Barrasso of Wyoming, either voiced concerns about Kennedy or declined to say they supported him in the immediate aftermath. Others have privately aired their worries directly to Trump and other White House aides, according to a person familiar with the discussions.
Senator Bill Cassidy (R-LA) questions HHS Secretary Robert F. Kennedy Jr. during a hearing of the Senate Finance Committee in Washington, on Sept. 4, 2025. – Francis Chung/Politico/AP
Cassidy, meanwhile, is locked in a tight race for reelection against multiple Republicans who have criticized the two-term senator for voting to convict Trump in 2021 during the Senate’s impeachment trial. That decision cost him the president’s endorsement, Trump allies say. Creating further headaches for the White House now could spur Trump to come out definitively against him, effectively ending Cassidy’s career in the deep-red state.
Multiple campaign operatives tracking the race say Cassidy can still hang on in the primary if Trump doesn’t endorse another candidate – and note that many Louisiana insiders are waiting to see if a different competitor jumps in: GOP Rep. Julia Letlow, a Louisiana conservative close to Trump who’s considering entering the race. Asked on Tuesday if she has made a decision on whether to enter, Letlow told CNN: “I have not.”
“The White House is watching,” said a Republican close to the White House, adding that Cassidy’s recent criticisms of Kennedy have already irritated aides around the president. “He’ll seal his own fate if he hasn’t already, if [the hearing] is perceived as unfair, if it’s perceived as one-sided, if it’s perceived as yet another opportunity to trash Kennedy.”
Cassidy, a former physician who has long advocated for vaccines and their safety, has downplayed the rift with Kennedy and has insisted that the Monarez hearing aims solely to get the facts of her ouster and the surrounding controversy over vaccine decision making. A spokesperson for the senator did not respond to multiple requests for comment for this article.
Some members of his own party view the hearing differently.
“I think that she didn’t go to the position with good faith. She did not go to the position willing to support the science,” Sen. Rand Paul of Kentucky said of Monarez. “I think it was right of her to leave the administration, and frankly, I’m glad she’s gone.”
Since casting a vote in favor of Kennedy’s appointment in exchange for commitments he said would ensure an “unprecedentedly close” working relationship, Cassidy has faced sharp criticism from public health advocates who fault him for enabling Kennedy’s disruptive agenda.
“Senator Cassidy drew lines in the sand, and when those lines were crossed, he did nothing,” said Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia, who had privately counseled Cassidy against voting for Kennedy in the run-up to his confirmation.
Health and Human Services Secretary Robert Kennedy Jr. leaves for a short break as he testifies before the Senate Finance Committee on September 4, 2025 in Washington, DC. – Andrew Harnik/Getty Images
The senator has won few fans in Kennedy’s orbit either, where allies quickly soured on him over his vaccine advocacy and some Kennedy advisers privately viewed Cassidy’s involvement with their work as an obligation rather than a true partnership.
Cassidy has nevertheless sought to avert a head-on confrontation with Kennedy that could force Trump to pick a side despite months of private frustration over the decision-making at HHS, declining to directly criticize the HHS secretary and wrapping his concerns in pro-Trump rhetoric — most recently arguing that Trump deserved a Nobel Prize for developing the Covid vaccines before pressing Kennedy over why he would now seek to restrict access to those shots.
Still, his decision to invite Monarez’ testimony a day before a panel of handpicked Kennedy allies considers changes to more vaccine recommendations was widely interpreted within Kennedy’s orbit as a direct effort to dent his agenda. It followed a testy exchange two weeks ago between them punctuated at one point by Kennedy’s curt dismissal: “Is this a question, Sen. Cassidy, or is this a speech?”
And while Kennedy advisers maintain there is always a chance they could reconcile, there is little immediate appetite to do so. In a sign of the divide, a public request Cassidy made last week that Kennedy endorse the whooping cough vaccine amid one of the worst outbreaks in Louisiana in recent history has gone unanswered.
“HHS is working closely with the state of Louisiana and its public health agency to monitor its pertussis caseload,” HHS spokesman Andrew Nixon said.
Nixon did not respond to a series of other questions from CNN.
Susan Monarez testifies during her confirmation hearing before the Senate Committee on Health, Education, Labor, and Pensions in the Dirksen Senate Office Building on June 25, 2025. – Kayla Bartkowski/Getty Images
Monarez’ account of her ouster — which occurred just months after Trump touted her credentials in nominating her to run the CDC — offers a critical backdrop to recommendations that Kennedy’s vaccine panel is expected to make later this week delaying the hepatitis B vaccine until age 4. The shot is typically given within one day of birth, as infected mothers can pass the virus to infants.
The prospect of altering that longstanding vaccine recommendation has dismayed Democrats and public health experts and prompted pushback from Cassidy, who has repeatedly emphasized the benefits of early vaccination and disputed misinformation about the shot.
That has left some unswayed: Paul said Tuesday that “he’s really wrong and uninformed” about early vaccination against the virus.
More unnerving to a wider array of GOP senators, though, is the potential that it could be a prelude to a fuller revamp of standard childhood vaccinations that remain broadly popular across partisan lines. In an interview earlier this week, Robert Malone, a vaccine critic who now sits on Kennedy’s advisory panel, said they had “four years to crank through” a planned comprehensive review of childhood vaccines.
Yet at least for now, Cassidy remains largely alone in trying to manage the fallout of a deteriorating relationship he once appeared to optimistically think might reunify people behind a common faith in public health.
“He’s in a very hard bind, a very conservative state in some ways, and he’s very senior and yet is living in the fear that Trump in one tweet can blow him out of the water,” said the person who has spoken to both Cassidy and Kennedy, characterizing the political tightrope that Cassidy is having to walk. “But in the long run, you have to believe.”
CNN’s Sarah Ferris, Arlette Saenz and Morgan Rimmer contributed to this report.
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