WASHINGTON — Hillary Rodham Clinton came face-to-face Oct. 22 with the Republican-led special investigation of the 2012 deadly attacks on a U.S. diplomatic mission in Libya, hoping to put to rest the worst episode of her tenure as secretary of state and clear an obstacle to her Presidential campaign.

Democrats have assailed the investigation as a ploy to derail Clinton’s White House bid.

Clinton, the front-runner to win the Democratic nomination, kicked off a long day of questioning with a plea that the United States maintain its global leadership role despite the threat posed to U.S. diplomats.

She hailed the efforts of the four Americans who died in the Sept. 11, 2012 attacks, including the first Ambassador in more than three decades, but told the House Benghazi Committee that the deadly events have already been exhaustively scrutinized.

The hearing comes at a moment of political strength for Clinton. On Wednesday, a potential rival for the Democratic nomination, Vice President Joe Biden, announced he would not join the race. Clinton also is riding the momentum of a solid debate performance last week. The Benghazi panel’s Republican chairman says he won’t call Clinton to appear a second time.

But Rep. Trey Gowdy of South Carolina started the hearing with a series of questions that he said remained unanswered: Why was the U.S. in Libya, why were security requests denied, why was the military not ready to respond quickly on the 11th anniversary of 9/11 and why did the Obama Administration change its story about the nature of the attacks in the weeks afterward?

Clinton addressed some of these matters in her opening remarks. She stressed a need for diplomats to advance U.S. interests in the world, even in dangerous places, and said perfect security can never be achieved.

The committee’s top Democrat, Rep. Elijah Cummings of Maryland, described the entire probe as a partisan campaign replete with implausible conspiracy theories.

The Republican criticism has included contentions by some lawmakers that Clinton personally denied security requests and ordered the U.S. military to “stand down” during the attacks, or that her agency was engaged in an elaborate gun-running scheme in eastern Libya.

None of these were substantiated in the independent Accountability Review Board investigation ordered by Clinton after the deadly incident, and seven subsequent congressional investigations.

Gowdy is engaged in his own balancing act, portraying the panel as focused on the facts after comments by fellow Republicans describing the inquiry as an effort designed to hurt Clinton’s Presidential bid.

Democrats have pounced on the remarks, and pointed out that the probe has now cost U.S. taxpayers more than $4.5 million and, after 17 months, has lasted longer than the 1970s Watergate investigation that brought down President Richard Nixon.

Thursday’s hearing was expected to last the whole day, with breaks. Clinton was certain to face questions about her use of a private email account and server while serving as President Barack Obama’s chief diplomat, another issue that has beset her campaign.

A new Associated Press-Gfk poll shows the public mixed on Clinton and Benghazi. Four in 10 say they neither approve nor disapprove of how she has answered questions about the attack, while 20 percent approve and 37 percent disapprove.

Americans also are divided on Clinton’s emails, which have raised security concerns. More than half of those polled view her use of a private server as a minor problem or no problem at all, compared with 1 in 3 who think it is a major problem. Nearly two-thirds of Republicans call it a major problem.

In the weeks after Benghazi, Clinton took responsibility for the incidents. In recent months, she has said her use of a private email server was a mistake.

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MATTHEW DALY and BRADLEY KLAPPER