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 first — 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 argues 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.
"They're going after us very big-time and in a very negative way," Sanders said Friday in an interview with The Associated Press. "And I wanted to make very clear that we will not be a doormat, we will not be attacked without responding. And my point was to focus on the issues where I thought she was lacking."
The conflict between the two flared this week ahead of the crucial April 19 New York primary. On Wednesday, 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."
Sanders said in the AP interview Friday that "by definition, she has a great deal of experience. No one can debate that. But I think in terms of judgment," he said, pointing to her vote to authorize the Iraq war, supporting past trade deals and allowing super PACs to support her campaign. "I think those are judgment calls that call into question whether she should be elected president."
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" — 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 has since softened his rhetoric. In an interview Thursday on "CBS Evening News with Scott Pelley" Sanders noted that Clinton "has years of experience. She is extremely intelligent" and said that if Clinton is the party's nominee, "I will certainly support her."
On Friday morning at a Manhattan town hall meeting broadcast on NBC's "Today Show," Sanders said "of course," Clinton was qualified to become president.
"I respect Hillary Clinton, we were colleagues in the Senate, and on her worst day she would be an infinitely better president than either of the Republican candidate," he 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."
Sanders has rejected some lines of attack against Clinton. During a Democratic debate in October, Sanders diffused the issue of Clinton's private email server during her time as secretary of state, saying "the American people are sick and tired of hearing about your damn emails!"
With over a week to go before the New York primary, the tension is only expected to get worse.