Clinton ally: Black lives 'don't matter much' to Sanders
David Brock, a longtime ally of the Democratic presidential candidate Hillary Clinton, blasted rival Bernie SandersBernie SandersGOP at decisive moment on Planned Parenthood Chaffetz: Threats against lawmakers should be taken seriously Assange bashes Dems: The party ‘is doomed’ MORE on Thursday, suggesting he doesn't care about black lives. 
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."
"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.
Sanders has cut into Clinton's lead nationally in a poll released earlier this week, which found him leading among young voters but Clinton holding substantial leads among older voters and blacks and Hispanics.