Hillary Clinton and Bernie Sanders are now arguing about race, and like many such arguments in campaigns, it has nothing to do with any substantive difference between them on policy issues. But the stakes could hardly be higher — indeed, it’s no exaggeration to say that if Sanders can’t find a way to win over large numbers of African-American voters, he will have virtually no chance of winning the Democratic nomination for president.

Which is why, when Sanders released an ad showing him amidst his many adoring supporters, Clinton ally David Brock, who runs about a hundred different super PACs and other organizations devoted to getting her elected (I exaggerate, but only slightly) gave an interview in which he said: “From this ad, it seems black lives don’t matter much to Bernie Sanders.” Because of course, if the crowd shots in his ad aren’t diverse enough, that must mean Sanders doesn’t care whether black people live or die. (Full disclosure: some years ago I worked for David Brock for a time.)

Naturally, the Sanders campaign was outraged, but Brock’s attack cleverly alluded to the period last summer and fall when Black Lives Matter activists were interrupting Sanders at speeches and pushing him to endorse their agenda. Sanders was the perfect target for those actions, because he’s a liberal eager to show African-Americans that he’s on their side, but also someone likely to make the kind of verbal slips that would allow them to criticize him.