Key Democratic players are worried that Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE ally David Brock could be hurting her image and hampering her chances of winning the presidency.

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In interviews over the past month, Clinton donors, fundraisers and operatives have told The Hill that the concerns about Brock’s comments, particularly some of his attacks on Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE, stretch all the way to the top of Clinton’s political machinery.

A leading figure in the Democracy Alliance, the liberal equivalent of the conservative Koch brothers’ donor network, said donors he associates with would like to put Brock “back in the can.”

“I have heard people express concern that what he does could be harmful generally” to the campaign, the donor said.

Longtime Clinton fundraiser Bill Brandt, an Illinois-based businessman and personal friend of Bill Clinton, said of Brock, “David is well meaning but I think perhaps like a zealot. He should keep it in check a bit. I don’t think this needs to be about tearing Bernie down. … It’s getting nasty and it doesn’t need to be.”

One Brock comment that drew backlash regarded his plans to raise questions about the 74-year-old Sanders’s health, seen by many as a low blow at the senator’s age and one that left Clinton vulnerable given attempts by conservatives to portray her as being in fragile health.

After news reports emerged that Brock was going to raise the issue of Sanders not having released his health records, Clinton campaign chair John Podesta shot off an unusual tweet on Jan. 17 to Brock: “Chill out. We're fighting on who would make a better President, not on who has a better Physical Fitness Test.”

Brock backed off from running a public campaign to pressure the Vermont senator to release his health records, but even talk of it was enough to exasperate Guy Cecil, who runs Priorities USA Action, the main super-PAC supporting the former secretary of State’s campaign on whose board Brock once sat.

Asked about his private complaints about Brock, Cecil, through a spokesman, did not deny that he vented about Brock's comments, but he suggested it was water under the bridge.

“Someone needs to find a new tree to bark up because Priorities has a great working relationship with American Bridge and David is a valued member of our Board,” Cecil said in the statement, referring to American Bridge 21st Century, a super-PAC founded by Brock in 2010.

Another comment of Brock’s that was seen as unhelpful in Clinton’s donor network was when he said in late January that “it seems black lives don't matter much to Bernie Sanders” because the Sanders campaign had used mostly white talent in a commercial.

Brock has long been a controversial figure in Washington. A right-wing operative in the 1990s and a tormentor of then-President Bill Clinton William (Bill) Jefferson ClintonD-Day for Trump: September 29 Trump job approval locked at 42 percent: Gallup If Trump doesn't know why he should be president again, how can voters? MORE, Brock did a public about-face, apologizing in 1998 to the Clintons for his dirt-shoveling.

He has since become their fiercest defender, now running a menagerie of allied liberal groups including Correct the Record, which provides opposition research and rapid response.

Clinton supporters are pleased about Brock’s conversion but worry that his hardball tactics risk doing more harm than good.

Brandt, summing up the off-the-record views of a number of Democratic donors, told The Hill, “I think that there’s a pride on Hillary and Bill’s part in taking someone who was once their worst enemy and seeing him taking on the righteousness of their cause.”

But, he added, “We just had a primary with the first Jewish candidate to win a race. … This is all good, and I think Bernie needs to be praised for having done it.”

Clinton donors, allied strategists and fundraisers also concede that Brock’s volatility needs to be weighed against his value, which is considerable. Brock is known as a prolific fundraiser, skilled opposition researcher and a fearless counter-puncher.

“He’s always been a controversial character and people grouse about him, but he produces great research, so there’s a balance," said Tony Podesta, the chairman of Podesta Group, a top lobbying firm he co-founded with his brother John, Clinton's campaign chair.

“He’s very effective and he now and then swings and misses … so I mean he doesn’t hit a home run every time but that would be true of Babe Ruth.”

Brock would agree. He thinks Democrats play too gently and cede the offensive too willingly to Republicans. He also freely admits that he says and does things that give some in Clintonland the jitters.

“One of the roles of Correct the Record is to do things that the campaign shouldn’t do or won’t do or can’t do, and one of those things is being more out front with some of the more pointed criticisms of Sen. Sanders,” Brock said in an interview.

“I understand that there are some people out there who are carping, but the reality is that in a campaign, and people around the campaign, there are always a range of views.”

But some of Clinton’s major donors worry that Brock is giving the Sanders campaign an opening — which it gleefully took — to smear the candidate for her association to Brock and his super-PAC. This is a particular vulnerability given Sanders is running a populist campaign against Wall Street and what he claims is a corrupt campaign finance system.

“[Clinton] should be ashamed of her association with Brock,” said Sanders’s campaign spokesman Michael Briggs.

One of Clinton’s biggest super-PAC donors explained that because of his past associations, Brock will never gain the full trust of some within the Clinton camp, no matter how fiercely he advocates on Clinton’s behalf.

The mega-donor believes that much of the sniping about Brock is due to personal hostility, which is inevitably exacerbated when campaigns hit rough waters, as the Clinton campaign is now after losing badly in New Hampshire.

“He rubs people up the wrong way,” the mega-donor said of Brock.

Asked whether the Clinton campaign worries that some of Brock's comments could hurt the campaign, deputy communications director Christina Reynolds said, “Throughout this campaign, Correct the Record has done a great job defending Hillary Clinton’s record and we are grateful for their tireless work defining the Republicans as extreme and out-of-date.”

Unless a major controversy erupts, Brock will remain one of Clinton’s key allies in 2016. He has plenty of high-profile defenders, and even some major super-PAC donors who don’t like what he says concede that if it weren’t Brock then somebody else would have to play that role for Clinton.

In the 24 hours after The Hill informed Brock about this story, three allies got in touch to say they had heard about the article and wanted to share their views.

Clinton donors Michael Kempner and Kenneth Jarin both offered unsolicited praise for Brock, saying he was an essential countervailing force against brutal attacks on Clinton.

“I think that the Sen. Sanders campaign is bringing up negative arguments, many of which I don’t think are accurate,” Jarin said. “I think David Brock can shed some light on some aspects of Sen. Sanders that will be good for voters to know.”

And an unsolicited email arrived from James Carville, a longtime Clinton friend and operative.

“Let me get this straight,” he wrote. “Folks are yapping about David Brock or Correct the Record being too aggressive. Give me a break.

“We are in the midst of a spirited Democratic primary campaign and a Supreme Court nomination looms in the near future that will have a lifetime of importance.

“It would seem to me we need to be very realistic about the stakes here in this election and bedwetting Democrats worried about surrogate intensity should simmer down just a tad.”

Read more from The Hill:

Brock: Sanders a 'typical politician'