Robert F. Kennedy Jr., President Donald Trump's nominee to head the Department of Health and Human Services, has faced a tough confirmation process and could find himself denied of his designated administration role by one Republican senator.

Sen. Bill Cassidy (R-LA), a former doctor who has taken issue with Kennedy's skepticism toward vaccines, has not openly declared himself a "No" vote on the nominee but did admit that he was "struggling" with the decision, NBC News reported.

That does not sit well with other Republicans who aim to confirm all of Trump's nominees, and the Louisiana senator, who is already facing an impending primary challenge for his vote to impeach Trump at the end of his first term, is facing sharp criticism and possibility of further political opposition to his quest to remain in the Senate.

Cassidy upset over Kennedy's vaccine skepticism

NBC News reported that Sen. Cassidy repeatedly hammered HHS nominee Kennedy over his long-held skepticism about vaccine safety, particularly his belief in a link between vaccines and autism in young children, and seemingly attempted to coerce him into denouncing those views during a confirmation hearing.

"Your past of undermining confidence in vaccines with unfounded or misleading arguments concerns me," Cassidy told Kennedy at one point. "Can I trust that that is now in the past? Can data and information change your opinion, or will you only look for data supporting a predetermined conclusion?"

Of his trust in the safety of vaccines, the senator said, "I can say that I’ve approached it using the preponderance of evidence to reassure and you’ve approached using selected evidence to cast doubt."

Cassidy undecided on Kennedy's nomination

At another point during the hearing, per NBC News, Sen. Cassidy asked Kennedy if he would reassure the public that there is no link between vaccines and autism if he was presented with convincing evidence of that conclusion, and the nominee replied affirmatively and even took it one step further.

"Not only will I do that, but I will apologize for any statements that misled people otherwise," Kennedy vowed. "I just want to pledge to you that I will never stick on a point if somebody shows me data that says I’m wrong."

The senator himself seemed unconvinced by the nominee's response, however, and urged at the conclusion of the hearing, "Man, if you come out unequivocally vaccines are safe, it does not cause autism, that would have an incredible impact."

"That’s your power. So what’s it going to be? Will it be using the credibility to support lots of articles, or will it be using the credibility to undermine?" Cassidy added. "I got to figure that out for my vote."

Potential for political consequences

According to The Hill, though Sen. Cassidy has not fully declared his opposition to Kennedy's nomination, unsubtle warnings have been issued by the White House and other Republicans about potential political consequences if he is the reason why Kennedy isn't ultimately confirmed.

"The linchpin, you know, on RFK is going to be Cassidy and he’s got a very very difficult choice," an unnamed Team Trump source told the outlet. "He is very likely to have a very difficult primary and he already … carries the weight on how he voted to impeach Trump."

Fox News reported that some GOP activists and elected officials similarly insinuated that ousting Cassidy from the Senate would be a top priority if he was responsible for undermining Kennedy's nomination.

Assuming Kennedy's nomination is eventually cleared by the Senate Finance Committee, as well as that all Senate Democrats stand united against him, the HHS nominee can only afford to lose the support of three Senate Republicans and still be confirmed with a tie-breaker vote from Vice President JD Vance.