Hillary Clinton and her supporters are blistering the Benghazi committee ahead of her much-anticipated testimony Thursday, repeatedly questioning the GOP-led investigative panel’s “credibility” as the former secretary of state gears up for a potentially confrontational appearance. 

On Wednesday, a super PAC supporting Clinton’s Democratic presidential campaign, Priorities USA, will begin running TV ads aimed at bolstering her image ahead of her appearance before the House Select Committee on Benghazi.

The effort marks the group’s first TV ad buy of the election cycle. But it is also just part of an all-out offensive that unexpectedly started Sept. 29 when House Majority Leader Kevin McCarthy suggested the committee -- created to investigate the 2012 terror attacks in Benghazi, Libya -- has hurt Clinton’s poll numbers. Within days, New York Republican Rep. Richard Hanna and a GOP committee investigator also suggested the committee was too focused on Clinton, giving her and campaign officials an opening to call the panel a partisan tool.

“This committee is basically an arm of the Republican National Committee,” Clinton said to applause during last week’s Democratic primary debate. “It is a partisan vehicle, as admitted by … Mr. McCarthy, to drive down my poll numbers.” A few days earlier, Clinton told NBC the committee was “set up … for the purpose of making a partisan, political issue out of the deaths of four Americans.” 

It's an allegation that Republican committee Chairman Trey Gowdy has adamantly denied, telling his Democratic committee counterpart as recently as Sunday that the committee "is not investigating Secretary Clinton" or the allegations surrounding her personal email use. 

Whether the pre-hearing charges will lead to fireworks Thursday remains to be seen. Gowdy appears to be at pains to show his committee is only interested in getting at the truth regarding the Benghazi attacks, while Clinton publicly casts the panel as a partisan outfit. Clinton showed visible frustration during her 2013 Benghazi-related appearance on Capitol Hill, where she asked "what difference, at this point, does it make" what motivated the attackers. The Democratic presidential front-runner surely is mindful that such an unguarded moment on Thursday could become fodder for GOP ads in the 2016 cycle. 

The committee itself was formed last year to investigate the Sept. 11, 2012, attack that killed Ambassador Chris Stevens and three other Americans at the U.S. diplomatic outpost in Benghazi, while Clinton was secretary of state.  

The 12-member bipartisan committee discovered this March that Clinton used a private server and email accounts for official business while in office, which has led to an FBI investigation, several other congressional probes and widespread concerns about whether her unusual setup resulted in national security breaches.

Still, Gowdy says the committee is focused on Benghazi. He and Republican committee member Rep. Mike Pompeo, of Kansas, indicated Sunday they have no intentions of closing the investigation and in fact have dozens more witnesses and more information, including new Stevens’ emails.

“The ambassador asked for more security, and it was ignored,” Bradley Blakemen, former deputy assistant to President George W. Bush, said Tuesday.

However, Clinton supporters and others have called for shuttering the 17-month-old committee -- arguing it’s a political sham and a $4.5 million taxpayer waste.

“If you want to get to the truth, you might want to broaden your reach as opposed to … for political reasons, just going after Hillary Clinton,” Democratic strategist David Mercer told FoxNews.com on Tuesday.

Critics have more recently noted that Republican committee members recently summoned long-time Clinton aide-de-camp Huma Abedin to testify while thus far not doing the same for then-Defense Secretary Leon Panetta, then-CIA Director Gen. David Petraeus and others.

Clinton campaign spokesman Brian Fallon told reporters last week that Clinton would still testify but that Gowdy’s inquiry now has “zero credibility left.”

The counter-attacks have more recently focused on Gowdy. The Washington Post last week found an alleged connection between him and the STOP Hillary PAC that ran a controversial Benghazi ad during the Democratic debate, resulting in Gowdy returning $2,000 in contributions.

The South Carolina Republican and former state and federal prosecutor recently told Politico that the past few weeks have been among “the worst in my life.” In response to Republican non-committee members critiquing their work, he said over the weekend that they should “shut up.”

The hearing Thursday is expected focus in large part on whether Clinton, who in 2013 testified before Congress on Benghazi, adequately responded to concerns by Stevens about security at the Benghazi outpost. 

Meanwhile, the Clinton campaign has produced several videos ahead of Thursday’s hearing including a five-minute highlight reel that touts Clinton’s "smart leadership” as secretary of state.