Martin O’Malley was booed at a liberal conference on Saturday when, after being interrupted by Black Lives Matter demonstrators, he said, “Black lives matter. White lives matter. All lives matter.” Within hours, the former Maryland governor apologized, saying on the digital show “This Week in Blackness,” “I meant no disrespect. That was a mistake on my part and I meant no disrespect. I did not mean to be insensitive in any way or communicate that I did not understand the tremendous passion, commitment and feeling and depth of feeling that all of us should be attaching to this issue.”

But O’Malley isn’t the only Democratic presidential candidate who has said “all lives matter.” Frontrunner Hillary Clinton, who didn’t attend Netroots, used the phrase in a speech last month at a historic black church five miles from Ferguson, Missouri. Describing her mother's life, Clinton said, “Her own parents abandoned her. By 14 she was out on her own, working as a housemaid. Years later, when I was old enough to understand, I asked her, ‘What kept you going?’ Her answer was very simple: Kindness along the way from someone who believed she mattered. All lives matter.”

There was some debate over whether she ever needed to clarify her remarks. Democratic strategist Donna Brazile didn’t see the need for it. 

Clinton has never explicitly apologized for saying “all lives matter.” Asked Monday if she regretted using the phrase and plans to apologize for it, her campaign sent a link to a Facebook comment by Clinton in response to a question by the Washington Post’s Wesley Lowery.

“All lives matter” has become a common conservative retort to Black Lives Matter, and activists (among others) object to the phrase because it obscures how black lives in particular are treated as less valuable in America. As Berkeley professor Judith Berkeley put it earlier this year:

Claiming that “all lives matter” does not immediately mark or enable black lives only because they have not been fully recognized as having lives that matter. I do not mean this as an obscure riddle. I mean only to say that we cannot have a race-blind approach to the questions: which lives matter? Or, which lives are worth valuing? If we jump too quickly to the universal formulation, “all lives matter,” then we miss the fact that black people have not yet been included in the idea of “all lives.”

Update: Another Democratic presidential candidate, Vermont Senator Bernie Sanders, has made remarks not unlike "all lives matter." In June, he told NPR's David Greene, "Black lives matter. White lives matter. Hispanic lives matter. But these are also not only police matters. They're not only gun control matters. They are significantly economic matters."