"The Clinton campaign has changed its tone, and I think they were pretty public about it," Sanders told NBC's "Today," suggesting Clinton's campaign was being more negative as the campaign shifts to New York ahead of the April 19 primary.

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"Here's the truth, I've known Hillary Clinton for 25 years. 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 candidates," Sanders said.

"She's qualified?" co-host Savannah Guthrie asked.

"Of course," Sanders responded.

It was a reversal by Sanders two days after he ticked off a list of issues that he suggested made Clinton " not qualified " to be president, including her 2002 vote for the Iraq War, money from Wall Street and support for trade agreements.

Sanders on Thursday defended his criticism of Clinton, pointing to a Washington Post headline as evidence that Clinton had first questioned his qualifications for the presidency.

That story was about Clinton's interview on MSNBC's "Morning Joe," where she repeatedly dodged when asked whether Sanders was qualified but suggested he "hadn't done his homework" on several issues.