Kennedy Jr. rejected characterizations of him as an anti-vaxxer in a Senate hearing Wednesday where senators will weigh his confirmation as Health and Human Services Secretary—as his former billionaire running mate threatens to fund primary challenges against lawmakers who vote against him.

RFK Jr. claimed he is not “anti-vaccine” and appeared unfamiliar with key aspects of healthcare insurance programs in his confirmation hearing.

While Mr. Kennedy, seeking the job of health secretary, has been vocal about vaccines and his desire to overhaul the nation’s diet, he has said very little about other issues.

The hearings were packed to capacity with protesters and fans, while the spillover crowd hung out in hallways and overflow rooms.

Democrats focused on Kennedy's anti-vaccine advocacy. Republicans probed the former Democratic leader on his abortion and agriculture views.