A spokesperson for the Bernie Sanders campaign on Thursday struck back at comments by a political operative who founded a super PAC backing Hillary Clinton and who suggested "black lives don't matter much" to the Vermont senator, saying the former secretary of state "should be ashamed of her association with" him.

The operative is David Brock, a former "nemesis" of the Clintons and former conservative journalist who now, as the Associated Press reports, "runs several super PACs aiding [Hillary Clinton's] candidacy."

Brock, who's previously described himself as a right-wing hit man, is also well known for his works in 1990s that sought to discredit Anita Hill's accusations of sexual harassment against Clarence Thomas, including his description of Hill as "a bit nutty and a bit slutty."

Brock's comments were in response to Sanders' new, one-minute ad featuring Simon and Garfunkel's 1968 song "America," which you can see below:

He told AP that it was a "significant slight to the Democratic base," adding, "From this ad it seems black lives don't matter much to Bernie Sanders."

But Sanders campaign spokesperson Michael Briggs responded by slamming Brock as a liar and mudslinger.

"Bernie Sanders, as everyone knows, has one of the strongest civil rights records in Congress," Briggs said in a statement. "He doesn’t need lectures on civil rights and racial issues from David Brock, the head of a Hillary Clinton super PAC. 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. He later was forced to apologize for his lies about her.

"Today," Briggs continued, "he is lying about Sen. Sanders. It’s bad enough that Hillary Clinton is raising millions in special-interest money in her super PACS. It is worse that she would hire a mudslinger like David Brock. She should be ashamed of her association with Brock."

The new ad, which begins airing Friday in Iowa and New Hampshire , has been criticized by other democratic analysts as being too white, while, as a New York Times reporter sees it, "Minorities are given prominence, presumably to address Mr. Sanders’s difficulties in attracting support among blacks and Hispanics."

The ad controversy comes as a new poll of Iowa voters shows Sanders with a "solid" 8-point lead over his rival in the early caucus state.