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.

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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.”