Sanders explained at a press conference in Philadelphia that he was responding to attacks from the Clinton campaign, which he says has been trying to “disqualify” him.

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“That was what was thrown at me,” he said, noting Clinton's strategy has been described as “disqualify him, defeat him and then unify the party later.”

Sanders, however, he did seem to soften his comments some by suggesting he was merely using the same tactic that she was. "This campaign will fight back" if attacked, he said. "This is not the type of politics that I want to get in." Sanders said Clinton started the fight over being qualified for the Oval Office and that he was responding in kind. He also blamed the media, which he accused of being uninterested in covering his calls to help the middle class. "I don't think I have to explain to the American people what Wall Street did to this economy," he said while talking about Clinton donors. "Are you qualified to be president of the United States when you're raising millions of dollars from Wall Street?"

Sanders pointed to a Washington Post story titled "Clinton questions whether Sanders is qualified to be president" in an effort to justify his response. The story reported on a "Morning Joe" interview in which Clinton repeatedly declined to answer a direct question about whether Sanders is qualified to be president.

At a rally on Wednesday night, Sanders shot back, saying Clinton is unqualified for accepting millions in donations from Wall Street, and supporting the war in Iraq and “disastrous” U.S. trade deals.

Clinton on Thursday morning declined to respond directly to Sanders' remarks, instead talking about unifying the Democratic Party before the general election.