WASHINGTON: Democratic presidential candidate Hillary Clinton deflected harsh Republican criticism of her handling of the deadly 2012 attack in Benghazi, Libya, at a testy hearing in Congress on Thursday that seemed unlikely to put a dent in the front-runner's campaign. At a sometimes bitter day-long hearing, the former secretary of state shrugged off Republican accusations that she ignored requests for security upgrades in Libya and misinformed the public about the cause of the attack by suspected Islamist militants that killed the U.S. ambassador and three other Americans in Benghazi. Clinton, 67, avoided the fray during several heated exchanges between Republicans and her Democratic allies, and stayed composed under aggressive questioning from Republican lawmakers. But hours of questioning uncovered no new revelations in a deadly incident that has been the subject of a half-dozen other congressional investigations and an independent inquiry. Clinton told the panel that it was 'personally painful' to be accused of ignoring security upgrades that could have saved the life of ambassador J. Christopher Stevens and three other Americans at the diplomatic compound. 'I've thought more about what happened than all of you put together,' she told the Republican-led special panel. 'I've lost more sleep than all of you put together. I've been racking my brain about what could have been done, should have been done.' The appearance before the Benghazi panel was a political test for Clinton, who has been on a hot streak since turning in a strong performance at last week's first Democratic debate and after Wednesday's news that her strongest potential challenger, Vice President Joe Biden, will not seek the Democratic nomination for the November 2016 election. Clinton defended her leadership in Libya as America's top diplomat and denied longstanding Republican allegations that she personally turned down requests to beef up security in Benghazi. 'I was responsible for quite a lot,' Clinton said. 'I was not responsible for specific requests and security provisions.' Clinton told the panel the attacks must not discourage US action globally and said the incident already had been thoroughly investigated. 'We need leadership at home to match our leadership abroad, leadership that puts national security ahead of politics and ideology,' Clinton said in her only early reference to the political controversy that has dogged the panel. Opinion polls show Americans deeply split along partisan lines over the probe. A Reuters/Ipsos poll this week found 35 percent of respondents viewed the Benghazi hearings as mostly or completely valid. The percentage among Republicans was 67 percent, independents 39.6 percent and Democrats 16.5 percent. 'A FISHING EXPEDITION' The panel has spent 17 months looking into the attacks that at the US mission compound. Clinton's long-awaited testimony was the most high-profile appearance yet before a committee that has already interviewed more than 50 witnesses. At one point, Clinton impassively stacked papers while Republican Chairman Trey Gowdy and senior Democrat Elijah Cummings argued loudly over Cummings' request that the closed-door testimony of Clinton friend Sidney Blumenthal before the committee in June be publicly released. Clinton listened intently, head in hand, as Gowdy heatedly questioned her about the constant emails she received from Blumenthal. Republicans noted that Stevens, the ambassador, did not even have Clinton's email address. 'You didn't need my email address to get my attention,' Clinton said.   Cummings said congressional Republicans set up the panel for a partisan witch hunt. 'They set them loose, Madame Secretary, because you're running for president,' he told Clinton, calling for an end to the 'taxpayer-funded fishing expedition.' He said the committee had spent $14.7 million of taxpayer money over 17 months. Republican Representative Peter Roskam told Clinton she was the chief architect of US policy in Libya and that 'things in Libya today are a disaster,' but Clinton said President Barack Obama made the final call on US Libya policy. Republican Representative Jim Jordan said Clinton had misleadingly implied after the attack that it was a reaction to an anti-Muslim video. Clinton, who denies suggesting the video was the cause, said the accusation had been proven false by other investigations. Clinton's appearance before the panel follows months of controversy about her use of a private home email server for her State Department work, a disclosure that emerged in part because of the Benghazi committee's demand last year to see her official records. GOWDY ON THE DEFENSIVE Gowdy, a former federal prosecutor, has been on the defensive over a series of comments from his fellow Republicans implying the committee's real aim was to deflate Clinton's poll numbers. 'Madame Secretary, I understand some people - frankly in both parties - have suggested this investigation is about you. Let me assure you it is not,' Gowdy told Clinton. 'Not a single member of this committee signed up for an investigation into you or your email system.' Clinton refrained from questioning the panel's motives, which she has done in recent public statements on the campaign trail. 'Despite all the previous investigations and all the talk about partisan agendas, I’m here to honour those we lost and to do what I can to aid those who serve us still,' she said. She said the emails being made public and examined by the committee did not encompass all of the work she did as secretary of state. 'I don’t want you to have a mistaken impression about what I did and how I did it,' she said. 'Most of my work was not done on emails with my closest aides, with officials in the State Department, officials in the rest of the government.' She cited communications through secure phone calls, in-person conversations and top-secret documents. The committee's Democrats, who may discuss abandoning the inquiry after Clinton's appearance, say they think there is little left to unearth on Benghazi. A 2012 report by a government accountability review board sharply faulted State Department officials for providing 'grossly' insufficient security in Benghazi, despite upgrade requests from Stevens and others in Libya.