David Brock, a former conservative, is now an ally of Hillary Clinton and is attacking her main rival for the Democratic nomination, Sen. Bernie Sanders. (Danny Johnston/AP)

David Brock swings hard.

He did it for the right in 1992, when he went after Anita Hill for accusing Supreme Court nominee Clarence Thomas of sexual harassment.

And he’s doing it now against Sen. Bernie Sanders (Vt.), who is threatening to defeat Brock’s more recent ally, Hillary Clinton, in the first two presidential nominating contests, in Iowa and New Hampshire.

Less clear is whether Brock’s shock-troop tactics are helping or hurting her.

Clinton began the race last spring as the prohibitive favorite to win the Democratic nomination. But Sanders leads her in polling in New Hampshire and is closing the gap in Iowa.

[Sanders prepares to challenge Clinton on Super Tuesday and beyond]

Brock has weighed in several times in the past week with harsh attacks against Sanders. Most recently, after Sanders released a feel-good ad in Iowa featuring thousands of images of his supporters — and set to the Simon and Garfunkel tune “America” — Brock noted not so gently that the overwhelming majority of the people in the ad were white.

“From this ad, it seems black lives don’t matter much to Bernie Sanders,” Brock told the Associated Press.

A spokesman for Sanders, who has sought to incorporate many of the goals of the Black Lives Matter movement in his campaign, called Brock a “mudslinger” and said Clinton “should be ashamed of her association” with him.

It was the third time in recent days that Brock had leveled such sharp criticism at Clinton’s main rival for the nomination. Last weekend, Sanders released his medical records and declared himself “very healthy,” after Politico reported that Brock was planning to demand the documents. And Tuesday, in an interview with the Wall Street Journal, Brock suggested that Sanders could not win the general election because “he’s a socialist. Think about what the Republicans will do with the fact that he’s a socialist in the fall.”

Sanders describes himself as a democratic socialist, arguing for economic equality and the right to such services as health care and education.

Although the tone from both sides has become more critical as the race has tightened before the Feb. 1 Iowa caucuses, the Sanders campaign has mostly bragged about his poll numbers and focused on the candidates’ policy differences.

[Sanders says he’s ‘very healthy’ amid reports that a Clinton ally is seeking his medical records]

Brock once employed these kinds of rhetorical takedowns in the service of conservatism. During the contentious nomination hearings for Thomas, Brock wrote a magazine article, which later became a book, attacking Hill’s character and raising doubts about the veracity of her accusations. He also published an article about allegations of sexual misconduct during Bill Clinton’s tenure as governor of Arkansas that led to “Troopergate” and a lawsuit by Paula Jones accusing Clinton of sexually harassing her. Testimony in that suit included revealing details about Clinton’s relationship with Monica Lewinsky that nearly derailed his presidency.

But in 1998, Brock penned an open apology to Clinton — and has since played the role of liberal crusader. He founded Media Matters for America, which describes itself as a watchdog for “correcting conservative misinformation in the U.S. media.” He has also started or works closely with several PACs and super PACs aimed at helping Hillary Clinton win the presidency, including Correct the Record, a rapid-response operation.

“David Brock is one of the worst practitioners in the dark arts of dirty politics,” said Michael Briggs, spokesman for Sanders’s campaign.

[Super PAC launches ‘Let’s Talk Hillary’ to reveal a softer side of Clinton]

“When he was a right-wing extremist, he tried to destroy Anita Hill, a distinguished African American professor,” Briggs said Friday. “He later apologized for his lies about her. Now he is lying about Sen. Sanders. It is unfortunate that Hillary Clinton would be associated with such a professional mudslinger.”

The Clinton campaign did not respond to requests for comment about Brock’s most recent criticism of Sanders’s ad. But Saturday, after the story about Brock’s threat to demand Sanders’s medical records, Clinton campaign chairman John Podesta chastised Brock via Twitter: “Chill out. We’re fighting on who would make a better President, not on who has a better Physical Fitness Test.”

Jamal Simmons, a Democratic political strategist with the Raben Group, a Washington consulting firm, said Brock’s observation about the lack of diversity in Sanders’s ad was fair game: “It wasn’t that he was wrong, but he put a little too much sauce on it.”

“Every campaign has shock troops, someone out there who is willing to say things and put the conversation in a certain direction, and I think that’s David’s strength for the Clinton campaign,” Simmons said. “The danger for the Clinton campaign is that so many voters think of her as being too political that when overly political things happen, it reinforces that negative. So the danger for her is that her negatives get reinforced more than the opponent gets defined. I think the big challenge for the Clinton campaign is they’ve got to try to dampen the enthusiasm for Bernie Sanders but also light more of a fire under her supporters.”

John Wagner in New Hampshire contributed to this report.