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WASHINGTON (CNN) -- Former Maryland Gov. Martin O'Malley took a newly confrontational tone towards Hillary Clinton on Thursday, suggesting her recent shifts on hot-button policy issues like gay marriage showed a lack of principles.

Clinton, who launched her presidential bid last Sunday, this week urged the Supreme Court to rule same-sex marriage constitutional and expressed support for giving undocumented immigrants licenses, both reversals of her previous positions.

"I'm glad Secretary Clinton's come around to the right positions on these issues," O'Malley told the Guardian during a forum at Harvard's Institute of Politics. "I believe that we are best as a party when we lead with our principles and not according to the polls.

"Leadership is about making the right decision, and the best decision before sometimes it becomes entirely popular."

He released a quick video just hours after Clinton announced her shift on gay marriage that hinted at criticism of the change, but prior to the comments to the Guardian had avoided a direct hit on the candidate.

O'Malley is also contemplating a presidential bid, and has been working to position himself as a progressive alternative to the former secretary of State in the Democratic primary. Those two issues offer him an appealing basis for a pitch to progressives, as O'Malley signed bills legalizing same-sex marriage and allowing undocumented immigrants to obtain licenses during his time as governor.

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