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 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 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.”

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.