He also called the 60-second ad, set to begin airing Friday in Iowa and New Hampshire, a “significant slight to the Democratic base.”
The spot, which is set to Simon & Garfunkel’s classic folk rock song, “America,” includes thousands of images of people from Sanders’s large-scale campaign rallies, as well as an expanding grid of faces of people who support him.
The vast majority are white, but there are minority voters featured as well. From the outset of his campaign, Sanders, a Vermont senator, has acknowledged he faces a challenge attracting black and Latino voters -- one that his campaign has been seeking to address.
Late Thursday, Sanders spokesman Michael Briggs shot back at Brock, calling him a “mudslinger” and a liar and saying Clinton “should be ashamed of her association” with Brock.
"Bernie Sanders, as everyone knows, has one of the strongest civil rights records in Congress,” Briggs said. “He doesn’t need lectures on civil rights and racial issues from David Brock, the head of a Hillary Clinton super PAC.”
Briggs pointed to Brock’s earlier career as a conservative journalist who wrote a sharply critical story in the American Spectator about Anita Hill, who accused now-Supreme Court Justice Clarence Thomas of sexual harassment.
“Twenty-five years ago it was Brock – a mud-slinging, right-wing extremist – who tried to destroy Anita Hill, a distinguished African-American law professor,” Briggs said.
With a tight race in Iowa, the first nominating state, Thursday's episode suggests a nasty 10 days could be ahead.
It was the second dust-up in a week featuring Brock and the Sanders campaign.
On Sunday, a news report surfaced that Brock was preparing to make an issue of Sanders’s age and call for him to release his health records.
That prompted Jeff Weaver, Sanders’s campaign manager, to call Brock’s reported plans “one of the most desperate and vile attacks imaginable.”
Brock later denied the report.