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.

Robert F. Kennedy Jr. has said vaccines are not safe. His support for abortion access has made conservatives uncomfortable.

Robert F. Kennedy Jr. responded to tough questioning during his Senate confirmation hearings, defending his stance on public trust in health agencies, abortion, and vaccines. Despite criticisms over his past anti-vaccine activism,

Kennedy’s hearing Wednesday to be the nation’s top health official quickly devolved into an argument over the questions he’s raised for years about vaccine safety.

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.