The demonstrators, who were mostly black, responded by booing him and shouting him down.
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Judith Butler, a philosopher at the University of California, Berkeley, recently explained why some find it offensive to respond to the "Black Lives Matter" movement with the "all lives matter."

"When some people rejoin with 'All Lives Matter' they misunderstand the problem, but not because their message is untrue. It is true that all lives matter, but it is equally true that not all lives are understood to matter, which is precisely why it is most important to name the lives that have not mattered, and are struggling to matter in the way they deserve," Butler said in an interview with The New York Times. "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.'"

O'Malley isn't the first Democrat to come under fire for the remark. Hillary Clinton was criticized in June for doing the same thing.