The second toughest job in the Trump administration isn’t vice president or secretary of defense or secretary of state. It’s not even attorney general, though that’s close. It’s secretary of Health and Human Services. That’s Robert Kennedy Jr.’s domain. It’s tough because Donald Trump has charged RFK Jr. with pursuing sea-change agendas in health care and nutrition. Kennedy is threatening mighty powerful interests in both arenas.

A thousand current and former HHS employees signed a letter callin...