DES MOINES, Iowa -? Bernie Sanders’ claim that Hillary Clinton isn’t qualified to be President landed with a boom this week. The blow was far from the firsa?? and won’t likely be the last ? from the candidate who pledged to stay away from negative campaigning.

The Vermont Senator kicked off his insurgent Presidential bid last year with a pledge to focus on issues over character attacks and boasted often that he’s never run a negative ad.

But for months Sanders has sharply criticized Clinton, slamming her for supporting the war in Iraq, for her record on trade and most aggressively for her lucrative paid speeches before Wall Street bankers.

While his tone has shifted as the race has grown more combative on both sides, Sanders’ campaign officials argue that he has kept his promise. They say he has focused his fire on policy and is simply fighting back against Clinton’s own attacks.

“Bernie Sanders decided yesterday that he wasn’t going to go into the New York primary and be run over by their campaign,” said Tad Devine, a senior adviser to Sanders’ campaign. “He responded in kind.”

The conflict between the two flared this week ahead of the crucial April 19 New York primary. On April 7, Clinton questioned Sanders’ truthfulness and policy know-how, though she avoided direct questions about whether he was qualified to be president.

Still, Sanders seized on the remarks at a rally that night, telling a crowd of thousands that Clinton has been saying that he’s “not qualified to be president.”

“I don’t believe that she is qualified if she is, through her super PAC, taking tens of millions of dollars in special-interest funds,” he said.

Clinton aides and supporters pushed back aggressively. A fundraising email sent out shortly after from Christina Reynolds, the Clinton campaign’s Deputy Communications Director, said Sanders had “crossed a line,” calling it a “ridiculous and irresponsible attack.”

The increased scrapping comes as the surprisingly competitive Democratic race heads into the high-stakes final contests. Sanders has been on a winning streak, but still must take 68 percent of the remaining delegates and uncommitted superdelegates to win the Democratic nomination. That would require blow-out victories in the upcoming primaries.

Steve McMahon, a Democratic strategist who advised Howard Dean’s 2004 Presidential campaign, said Sanders is unlikely to win the nomination unless he can win “big states by big margins” a?? beginning with New York, the state Clinton represented in the Senate.

“For Hillary Clinton, this is about bragging rights. For Bernie Sanders, this is about survival,” he said. McMahon added that Sanders’ comments on Clinton’s qualifications was an “authentic reaction” to the situation, but “it was not accurate.”

“Trying to prosecute an argument that she’s not qualified to be president is ridiculous and it’s a losing argument,” he said.

Sanders softened his rhetoric in an interview April 7 on CBS Evening News with Scott Pelley.

“What I said was in response to what she has been saying. Washington Post headline, quote ‘Clinton Questions Whether Sanders is Qualified to be President.’ I thought it was appropriate to respond.”

He said Clinton “has years of experience. She is extremely intelligent” and said that if Clinton is the party’s nominee, “I will certainly support her.”

He also gave her some wiggle room on Iraq. “Of course she doesn’t bear responsibility” for Iraq war victims,” Sanders said. “She voted for the war in Iraq. That was a very bad vote in my view. Do I hold her accountable? No.”

Sanders wife Jane, appearing on MSNBC, said of her husband: “Bernie has moved on.”

But on Friday morning, Sanders said on MSNBC’s Morning Joe that he was forced to react more fiercely to Clinton statements in recent days because her campaign decided to “go negative” against him in the wake of his string of recent primary season victories.

He said he was being “attacked day after day after day” and said “what am I supposed to do? We’ve got to fight back.”

“Let’s get back to the issues,” Sanders said.

Clinton’s campaign has grown increasingly frustrated with Sanders’ attacks, particularly around campaign finance and Wall Street, which they say amount to character criticisms.

They have amped up their own rhetoric in recent days, hitting him for being weak on gun control and trying to pit him against the families of children murdered in the Sandy Hook school shootings.

Sanders volunteer Brenda Brink, from Huxley, Iowa, said Sanders was doing what he needed to do.

“If you want to call it negative, I call it politics,” said Brink, 58. “He’s not going to lay down and let it pass and no one really wants him to. It’s a fight.” With over a week to go before the New York primary, the tension is only expected to get worse.

(CATHERINE LUCEY and KEN THOMAS)