Democratic presidential candidate and former Secretary of State Hillary Clinton testifies before the House Select Committee on Benghazi October 22, 2015 on Capitol Hill in Washington, DC. Getty Images

Democratic presidential candidate Hillary Clinton passed a tough political test on Thursday, calmly deflecting harsh Republican criticism of her handling of the deadly 2012 attack in Benghazi, Libya, during a testy 11-hour hearing in Congress. In testimony that stretched deep into the night, the former secretary of state rejected 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, stayed out of the political fray during several heated arguments between Republicans and her Democratic allies and remained composed under aggressive questioning from Republican lawmakers. Read More The long hearing 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 said 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 critical hurdle 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. Even some Republicans said Republican lawmakers had swung at Clinton and missed with their aggressive questioning. "They forget Secretary Clinton has been dealing with hostile committees longer than most of them have been in politics at any level," Texas-based Republican strategist Joe Brettell said. Clinton not responsible for security Representative Trey Gowdy, chairman of the special panel, acknowledged to reporters afterward that Clinton's testimony was not significantly different than her previous testimony on the incident. 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 Read MoreTiming of Movie About Benghazi Attack Could Test Clinton inIowa Caucuses "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 U.S. 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 a veiled 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. The panel has spent 17 months looking into the attacks at the U.S. 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 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.