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.

If approved, Kennedy will control a $1.7 trillion agency that oversees food and hospital inspections, hundreds of health clinics, vaccine recommendations, and health insurance for roughly half the country.

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

During confirmation hearings, Robert F. Kennedy Jr. spoke to SNAP and his ideas for integrating nutritional health into federal assistance programs.

As he seeks to lead the health department, Mr. Kennedy wore a thin tie dotted with feathered creatures to cap a classic suit.

Robert F. Kennedy Jr. faced intense scrutiny about previous statements at his first confirmation hearing for DHHS secretary.