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.

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.

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.