One Republican, John Fleming, has already jumped in to challenge Cassidy for the party’s Senate nomination next year. Fleming, who served four terms representing a western Louisiana district in the House, cited Cassidy’s impeachment vote in announcing his bid in December.

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.