Joe Kennedy III on Friday denounced his cousin Health and Human Services (HHS) Secretary Robert F. Kennedy Jr. as "a threat to the health and wellbeing of every American" in a blistering social media post.The rare public rebuke comes one day after RFK Jr.s contentious Senate hearing, deepening a family feud that has fractured one of Americas most famous political dynasties.Kennedys statement, viewed more than 1.2 million times, opens the curtain behind the extraordinary rift inside the political dynasty.Once synonymous with Democrat unity, the Kennedys are now split over RFK Jr.s embrace of President Donald Trump and his appointment to HHS Secretary under a "Make America Healthy Again" agenda. Joe Kennedys demand that his cousin resign marks a dramatic escalation of the feud.INTO THE ARENA: HOSTILE DEMOCRATS, SKEPTICAL REPUBLICANS TEAR INTO KENNEDY ON THE HILLIn 2024, five of RFK Jr.s siblings issued a joint statement blasting his endorsement of Trump as "a betrayal of the values that our father and our family hold most dear. It is a sad ending to a sad story."Caroline Kennedy later urged senators to block her cousins cabinet nomination, writing in a January 2025 letter that he was a "predator" who was "addicted to attention and power" and "unqualified" to lead HHS.On Sept. 4, RFK Jr. appeared in a hearing before the Senate Finance Committee. Sen. Ron Wyden, D-Ore., accused him of firing CDC scientists and replacing them with "cranks" and "conspiracy theorists" who were "endangering children, leaving parents confused and scared."TRUMP STANDS BY RFK JR. AFTER HEATED SENATE HEARING: I LIKE THE FACT THAT HES DIFFERENTAs Joe Kennedy wrote in his post, "None of us will be spared the pain he is inflicting Those values are not present in the Secretarys office. He must resign."His cousin Jack Schlossberg, President John F. Kennedys grandson, piled on in real time, mocking RFK Jr.s testimony: "RFK LOSER is choking so badly LIVE."With Robert F. Kennedy Jr.s controversial tenure in Trumps cabinet, the Kennedy dynastys divisions are now spilling into public view with some of the sharpest blows coming from within.CLICK HERE TO GET THE FOX NEWS APPThe Department of Health and Human Services did not immediately respond to Fox News Digital's request for comment.