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.

Sen. Bill Cassidy of Louisiana has emerged as a central figure in the confirmation of Robert F. Kennedy Jr., President Donald Trump's nominee for health and human services secretary.