The Senate Finance Committee will vote on Tuesday on whether to advance Robert F. Kennedy Jr.’s controversial nomination to lead the Department of Health and Human Services under President Donald Trump.

It would take just one Republican to oppose Kennedy for his nomination to be potentially sunk, assuming all Democrats vote against him — as is expected.

All eyes will be on Sen. Bill Cassidy, a Louisiana Republican and longtime physician who, during last week’s hearings, expressed deep concerns about the impact of Kennedy’s past comments casting doubt on vaccines, including saying on a 2023 podcast that “no vaccine is safe and effective.”

Cassidy told Kennedy he was “struggling” with his nomination as those proceedings came to a close. The two spoke more over the weekend, according to one person familiar with the discussion, though it’s unclear what was said.

On Monday, Cassidy would not engage with questions on that conversation or on how he will vote on Tuesday.

“I really am not discussing RFK, I just keep saying that,” Cassidy told reporters.

While Kennedy’s nomination hinges on a single Republican vote in the committee, it’s possible the Senate Finance Committee could hold a second vote to send the nomination to the floor. Such a move would enable the full Senate to decide if Kennedy should be confirmed.

If Kennedy fails to get the support he needs to be reported favorably out of the Senate Finance Committee, there are two other options. The panel could report his nomination out of committee unfavorably or without recommendation, though that, too, would require a majority vote. Or, Senate Majority Leader John Thune could try to advance his nomination on the floor — but that would require an even higher threshold of 60 votes, which Republicans are not likely to get.

Trump said on Monday he’s called senators who have concerns about Kennedy. Vice President JD Vance has also been quietly lobbying senators to line up behind Kennedy, ABC News previously reported.

Questions have continued to swirl around Kennedy’s views on vaccines. He said several times during the hearings last Wednesday and Thursday that he supports vaccines and is not “anti-vaccine” but “pro-safety.”

However, Kennedy has openly questioned the widespread administration of both measles and polio vaccines, and has falsely linked the former vaccine to autism, despite several high-quality studies finding no such link.

He also pointed to a flawed paper to suggest there is evidence to claim that vaccines cause autism. Cassidy, the top Republican on the Senate Committee on Health, Education, Labor and Pensions, said he saw problems with the paper’s methodology upon first look.

Kennedy also cast doubt on the lifesaving benefits of the COVID-19 vaccines, saying he doesn’t think that “anybody” can say the vaccines saved millions of lives.

A 2022 study from the Yale School of Public Health and University of Maryland Medical School estimated the vaccine saved 3 million lives and prevented 18 million hospitalizations.

Senators were also befuddled by comments Kennedy made in the past. Democratic Sen. Michael Bennet of Colorado read aloud alleged past comments made by Kennedy, including unfounded claims about transgender children.

Kennedy denied making such comments despite repeated an unfounded conspiracy theory during an episode of his podcast in 2022, suggesting endocrine disruptors, including phthalates — which make chemicals more durable — and pesticides, can influence sexual orientation or gender identity.

ABC News’ Anne Flaherty and Will McDuffie contributed to this report.