But by Thursday night, the Vermont senator said he didn't want to engage in a tit-for-tat with Clinton, telling CBS News they should be "debating the issues facing the American people."
The controversy began late Wednesday when Sanders discussed Clinton's suitability for the White House. He cited a headline in The Washington Post
as evidence that Clinton's campaign was questioning his qualifications, warranting his response.
"The Washington Post had a headline that said 'Clinton questions whether Sanders is qualified to be president.' That was what was thrown at me."
He also cited a CNN report, "Clinton strategy: Defeat Sanders, unify party"
as further confirmation that the Clinton campaign started the attacks over qualifications.
"I believe the Clinton campaign told CNN that their strategy is, 'we go into New York and Pennsylvania. Disqualify him, defeat him and unify the party later,'" he said.
Speaking at a news conference in Philadelphia on Thursday, Sanders said, "They're going to question my qualifications, well I'm going to question theirs."
Clinton herself has never said Sanders isn't qualified to be president. When asked Wednesday on MSNBC if she thought Sanders was "ready to be president," she said: "I think he hadn't done his homework and he'd been talking for more than a year about doing things that he obviously hadn't really studied or understood, and that does raise a lot of questions."
"Really what that goes to is for voters to ask themselves can he deliver what he's talking about," she had said, referring to an extensive interview Sanders had given with the New York Daily News
during which he struggled to answer policy questions related to his signature issue -- reforming Wall Street -- and other topics such as gun control and foreign policy.
By Thursday morning, Clinton laughed off questions about Sanders' assertion that she isn't "qualified" for the presidency.
"It's kind of a silly thing to say," she told reporters in New York. "But I'm going to trust the voters of New York who know me and have voted for me three times."
At his news conference, Sanders said he was trying to run an "issue-oriented campaign" and blamed "the media" for taking things off course.
"As we can see, the media is not very interested in wage decline in the middle class
, that's not what you're interested in, but we have tried in rally after rally to talk about the most important issues facing the American people," he said.
He then repeated on Thursday the same swipes on Clinton he cited the night before.
"My response is if you want to question my qualifications, then maybe the American people might wonder about your qualifications Madame Secretary," he said.
Sanders added: "When you voted for the war in Iraq, the most disastrous foreign policy blunder
in the history of America, you might want to question your qualifications. When you voted for trade agreements
that cost millions of Americans decent paying jobs, and the American people might want to wonder about your qualifications. When you're spending an enormous amount of time raising money for your super PAC from some of the wealthiest people in this country, and from some of the most outrageous special interests ... Are you qualified to be president of the United States when you're raising millions of dollars from Wall Street
whose greed and recklessness helped destroy our economy?"
Sanders' remarks rankled Clinton's aides, with many arguing it shows Sanders' campaign growing desperate in the face of growing odds to win the Democratic nomination. Clinton's aides were outraged late on Wednesday night, when they gathered for a conference call about the change in tone.
"Hillary Clinton did not say Bernie Sanders was 'not qualified.' But he has now - absurdly - said it about her. This is a new low," campaign spokesman Brian Fallon tweeted.
Asked by CBS News' Charlie Rose Thursday night if he was getting into a "tit-for-tat"
with Clinton, Sanders defended the latest round of blasts.
"We should not get into this tit-for-tat. We should be debating the issues facing the American people," Sanders said. "All I am saying, if the people are gonna attack us, if they're gonna distort our record, as has been the case time and time again, we're gonna respond."
Rose asked Sanders if it was going too far to suggest Clinton bears responsibility for deaths in the Iraq war.
"Do I bear responsibility for the tragedy and the horrors of Sandy Hook? So, you know, let's get off of that," Sanders replied. "Of course she doesn't bear responsibility. She voted for the war in Iraq. That was a very bad vote, in my view. Do I hold her accountable? No."
The latest Quinnipiac poll
of New York Democrats finds Clinton beating Sanders 54% to 42%. That survey came out March 31, several days before Sanders won the Wisconsin primary
. In fact, Sanders has won seven of the last eight Democratic contests, though Clinton has a commanding lead among delegates.
In addition to a trove of delegates New York is an important symbolic contest. Sanders was born in the Empire State, and New York City has been at the center of the national political battle over income inequality -- a signature issue for the Vermont senator. But Clinton represented the state in the Senate, and her campaign headquarters is based in Brooklyn.
The two candidates will face off in a debate in New York on April 14
, hosted by CNN and NY1.