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. 
 
 
 
Sanders campaign manager Jeff Weaver suggested on MSNBC that viewers read the transcript of Clinton's "Morning Joe" interview Wednesday, saying of Clinton's campaign, "I think it's pretty clear what their strategy is."
 
Weaver said the Sanders campaign wanted to focus on issues like trade and income inequality, "but if the Clinton campaign wants to engage in a more bare-knuckle kind of approach, we're happy to do that as well."
 
Weaver accused Clinton of running a “smear campaign” centered on "innuendo and unfair attacks."
 
“If they want to engage in that kind of a campaign, they’re going to see how a real New Yorker fights back,” he said.
 
“He’s the son of Brooklyn, born in New York. He knows how the rough and tumble of New York, and if that’s the kind of campaign they want to run, he’s fully prepared to run it.”

He pointed to Sanders's success with small-dollar donations and contrasted it with Clinton's support from wealthy donors.

"You know, she's really made a deal with the devil, and the devil always wants his due. So that time will come."

Jonathan Easley contributed.

Updated at 10:04 a.m.