Rep. Trey Gowdy (R-S.C.). (Bill O'Leary/Washington Post)

Almost from the moment he was named chairman of the House investigation into the 2012 attacks on U.S. diplomats in Benghazi, Libya, Rep. Trey Gowdy (R-S.C.) has been insisting that he would keep the probe above the political fray.

“There are certain things in our culture that have to transcend politics, and I don’t mean to sound naive, but the murder of four fellow Americans and an attack on a facility that is emblematic of our country should transcend politics,” Gowdy said on Fox News Channel the day after he was tapped to lead the Select Committee on Benghazi.

In the 17 months since, his pleas have repeatedly fallen on deaf ears, deeply complicating Gowdy’s task as he prepares for the most closely scrutinized moment of the investigation: Thursday’s hearing featuring former secretary of state Hillary Rodham Clinton.

Republicans ranging from the sitting House majority leader to grass-roots activists and a prominent GOP operative have seized on the Benghazi attacks in bids to derail Clinton’s presidential campaign, emboldening Democratic critics.

As Democratic presidential candidate Hillary Clinton prepares to testify before the House Select Committee on Benghazi, here's a breakdown of what happened from the attack on Sept. 12, 2011 to the current political controversy. (Sarah Parnass/The Washington Post)

“I think we should take the chairman at his invitation, which is judging the committee by its work, and its work has been obsessively focused on Secretary Clinton,” said Rep. Adam B. Schiff (D-Calif.), a panel member. “I think that ultimately Benghazi is going to become synonymous with the abuse of congressional process.”

And the politicization of the probe has forced Gowdy, a former state and federal prosecutor chosen for his reputation as a careful investigator, to repeatedly play defense to preserve the committee’s credibility.

On Friday, Gowdy’s campaign returned three donations after The Washington Post inquired about links to a political action committee that aired a controversial ad about the Benghazi attacks during last week’s Democratic presidential debate.

The ad produced by the Alexandria, Va.-based Stop Hillary PAC showed images of the four Americans killed in Benghazi, including U.S. Ambassador J. Christopher Stevens, as a narrator spoke in their voice: “Dear Hillary Clinton, I’d like to ask you why you ignored calls for help in Benghazi and then four Americans were murdered. . . . But Mrs. Clinton, I can’t.”

The PAC’s treasurer, Dan Backer, previously served as treasurer of a now-defunct fundraising committee affiliated with Gowdy. He is the treasurer of three other PACs — the Conservative Action Fund, the Special Operations Speaks PAC and the Tea Party Leadership Fund — that each donated $2,000 to Gowdy’s campaign on the same day in May.

Those donations were returned Friday, said Jamal Ware, a spokesman for the Benghazi committee. Gowdy “has made every effort possible” to keep the investigation above the political fray, he added.

Rep. Trey Gowdy (R-S.C.) will lead the House committee investigating the 2012 attack on the U.S. consulate in Benghazi. Here are some of Gowdy's most memorable comments on the aftermath of the attack. (Jackie Kucinich/TWP)

“The memories of our lost fellow citizens should not be used for fundraising, nor should their families’ suffering be exploited,” Ware said. “If you hear Trey Gowdy, Benghazi and donate in the same message, Mr. Gowdy would personally encourage you to ignore the request.”

Backer said in an interview that he was not inclined to heed Gowdy’s concerns about politicizing the Benghazi attacks.

“I have a tremendous amount of respect for the guy,” he said. “But the purpose of the Benghazi ad that Stop Hillary PAC ran has nothing to do with Trey Gowdy. It has to do with holding the people responsible for this awful tragedy accountable for their mistakes.”

In April, the Stop Hillary PAC used Gowdy’s name and image in a fundraising solicitation calling on donors to “Support Trey Gowdy & Continue the Select Committee on Benghazi.”

Gowdy’s personal lawyer sent a letter to the group demanding that it “immediately cease and desist from any and all use of Rep. Gowdy’s name, image or likeness in any manner.” Gowdy refused a donation from the Stop Hillary PAC in May, Ware said, and he refused a July donation from another PAC linked to Backer.

The PAC raised nearly $800,000 from January through June, according to its most recent federal disclosure. Most of that has been spent on direct mail and consulting fees — including $31,520 to Backer’s own firm, DB Capitol Strategies.

But recent filings show that the group spent about $100,000 this month on a national ad campaign opposing Clinton — and it reported conducting $10,000 worth of automated phone calls last month in support of Gowdy. Backer said the calls were made in Gowdy’s district to counter attacks from Democrats on the Benghazi committee.

Backer’s role in Gowdy’s former “leadership PAC,” a fundraising vehicle commonly used to generate goodwill and influence by doling out money to favored colleagues and causes, represents another tie between the men.

Backer filed the papers in August 2013 establishing Themis PAC, named for the Greek goddess of justice, as well as the papers disbanding it last year, shortly after the Benghazi committee was established.

Both Backer and Ware dismissed that relationship as immaterial to the Benghazi probe. Backer said he did little besides file paperwork with campaign regulators and met Gowdy only once in passing after the PAC was shuttered. Ware said Themis PAC never had Gowdy’s full attention and support and was closed after raising only $5,000.

But the links had the Benghazi panel’s ranking Democrat, Elijah E. Cummings (Md.), fuming after they were disclosed Monday in The Washington Post: “These are shocking new revelations that directly contradict the promise we made to the families of the four brave Americans,” he said in a statement.

Besides returning checks, Gowdy has canceled several fundraising appearances since becoming chairman to distance himself from the appearance of political taint. But his toughest task has been distancing the probe from the claims of fellow Republicans.

He was forced to quell a firestorm after Majority Leader Kevin McCarthy (R-Calif.) suggested in recent interviews that the Benghazi panel was created with an eye toward undermining Clinton’s presidential bid.

”Everybody thought Hillary Clinton was unbeatable,” McCarthy told Fox News host Sean Hannity. “But we put together a Benghazi special committee, a select committee. What are her numbers today? Her numbers are dropping.”

And last week, Rep. Richard L. Hanna (R-N.Y.) told a radio talk show host that “there is a big part of this investigation that was designed to go after people — an individual, Hillary Clinton.”

In a Sunday interview on CBS’s “Face the Nation,” Gowdy lashed out — and not only at his Democratic critics: “I have told my own Republican colleagues and friends, shut up talking about things that you don’t know anything about. And unless you’re on the committee, you have no idea what we have done, why we have done it and what new facts we have found.”