Brock, who heads several groups aiding Clinton's bid, offered the accusation while criticizing a new ad from Sanders that Brock said was a "significant slight to the Democratic base."

ADVERTISEMENT

"From this ad it seems black lives don't matter much to Bernie Sanders," Brock told The Associated Press

A Sanders spokesman shot back Thursday at Brock, describing him to the Associated Press as a "mud-slinging, right-wing extremist" decades ago who is now "lying about Sen. Sanders."

Sanders's ad released earlier on Thursday painted a rosy picture, with a series of clips showing an overwhelmingly white crowd applauding Sanders set to the track "America" from Simon and Garfunkel.

Clinton and Sanders have thrown significantly sharper rhetorical attacks at each other leading up to ballots being cast in Iowa and New Hampshire in early February, where they're each in striking distance.