Robert F. Kennedy Jr. 's bid to be the nation's top health official is uncertain after a key Republican joined Democrats to raise persistent concerns over the nominee's deep skepticism of routine childhood vaccinations that prevent deadly diseases.

Sen. Bill Cassidy, the top Republican on the Senate Committee on Health, Education, Labor and Pensions, concluded Thursday's hearing by saying he was "struggling" with the nomination due to Kennedy's vaccine positions. Kennedy notably refused to say vaccines don't cause autism as he faced pointed question from lawmakers.