The race for the Democratic nomination is intensifying as candidates cast doubt over each other's qualifications to be president at overlapping press conferences.
Sanders told journalists in Philadelphia Thursday that he raised questions about Clinton's qualifications to be president because "that was what was thrown at me."
"If Secretary Clinton thinks... that we're not gonna fight back ... they can guess again," Sanders said.
The Vermont senator questioned whether Clinton is "qualified" to be president Thursday after she criticized his record and his preparedness for the job.
Clinton, campaigning in New York, sought to shift attention to her Republican opponents, saying "I will take Bernie Sanders over Donald Trump or Ted Cruz any time, so let's keep our eye over what's at stake in this election."