WASHINGTON>> Robert F. Kennedy Jr.’s bid to be the nation’s top health official is uncertain after a key Republican joined Democrats to raise persistent concerns over the nominee’s deep skepticism of routine childhood vaccinations that prevent deadly diseases.

Sen. Bill Cassidy, a Louisiana Republican, ended a three-hour confirmation hearing Thursday by telling Kennedy that he was “struggling” with his nomination and might call him over the weekend, though he did not say how he would vote.

Cassidy, a liver doctor who has regularly encouraged his constituents to vaccinate against COVID-19 and other diseases, implored Kennedy several times to reject theories that vaccines cause diseases like autism. Kennedy’s refusal to do so clearly troubled Cassidy.

“If there’s any false note, any undermining of a mama’s trust in vaccines, another person will die from a vaccine preventable disease,” Cassidy said.

The senator, who is up for reelection next year, laid out the dilemma before him, as a doctor who has seen how vaccines can save lives and as a Republican who is aware of Kennedy’s formidable support and wants to help President Donald Trump advance his policies.

Cassidy is no stranger to these predicaments and the outcry they might spark — he voted to convict Trump on impeachment charges after his first term in 2021.

Kennedy’s vaccine views could jeopardize his standing with a few crucial Republicans and has certainly not helped him win over any votes among Democrats in his bid to become health secretary. If all Democrats reject Kennedy’s nomination, he can only afford to lose three Republican votes.

Kennedy will also have to win over the swing votes of Sens. Susan Collins, Lisa Murkowski and Mitch McConnell, who have raised concerns about Kennedy and also voted against Trump’s defense secretary nominee.

Any Republican considering a “no” vote will face a maximum pressure campaign from Trump to line up and confirm his nominees. When others have expressed reservations about supporting nominees, they have met a coordinated campaign of political threats from Trump’s allies. Kennedy’s “Make America Healthy Again” movement has also encouraged followers to overwhelm the inboxes and phone lines of senators who may waffle.

Cassidy directly asked Kennedy if he would unequivocally reassure parents that the hepatitis B and measles vaccines do not cause autism. Kennedy would not.

Aside from Cassidy, Republicans on the health committee remained friendly to Kennedy. Republican Sen. Tommy Tuberville of Alabama told the hearing that his granddaughter, due in the coming weeks, would not “be a pincushion” when it came to vaccines.