Republican Sen. Bill Cassidy of Louisiana, a former physician who has spent his career touting the safety of vaccines, will likely determine whether Robert F. Kennedy Jr., a vaccine skeptic, becomes the country’s next Health and Human Services secretary.

The fate of Robert F. Kennedy Jr.’s bid to run HHS may rest with a Republican wrestling with his training as a physician and his instincts as a politician.

Bill Cassidy is deeply torn over what to do about Robert F. Kennedy Jr. As a doctor who once prescribed life-saving vaccines, Cassidy is troubled about putting someone in charge of the nation’s health agencies who has claimed, in defiance of the scientific consensus, that the shots are dangerous.