Health Secretary Robert F. Kennedy Jr. and his MAHA movement are out for payback, setting their sights on unseating a fierce adversary — Republican Sen. Bill Cassidy — in next week's Louisiana primary. Why it matters: Kennedy and his supporters view the physician-turned-senator as an avatar for the medical establishment they're determined to upend. The big picture: Cassidy, who chairs the Senate HELP Committee, has been openly skeptical of Kennedy's bid to reform health policy. After providing a pivotal vote to confirm Kennedy, Cassidy has pushed back on Kennedy's efforts to scale back vaccine recommendations and reshape the Centers for Disease Control and Prevention. He has also challenged Kennedy's claims that environmental toxins are a major driver of chronic disease. The most recent flare-up came last week, when Cassidy helped torpedo Casey Means' nomination for surgeon general. Means, a health influencer, is a close Kennedy ally and a major figure in the MAHA movement.…