He asked Kennedy to put aside his decades of voicing skepticism about vaccine safety and use his celebrity to encourage parents to vaccinate their children.

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.