If confirmed, Hagel will be the first defense secretary to have served all of his military career as an enlisted soldier. He was an Army sergeant in Vietnam, where he was wounded, and said on Thursday that his war experience was an influence in his life.
"I'm not shaped, framed, molded, consumed by that experience, but it's part of me," Hagel said, adding that he thought it would be a positive to have the defense secretary for the first time be someone "who understands the reality and consequences of war."
A sharp exchange on Thursday came when Sen. John McCain criticized Hagel's opposition to the troop surge in Iraq by the Bush administration and a similar move by Obama in Afghanistan. Both were crucial wartime decisions made by policymakers.
McCain, a former naval aviator and prisoner of war in Vietnam, said Hagel was wrong on both counts. The Arizona Republican said that he and Hagel, who once were close political allies and personal friends, had "fundamental differences" on important issues.
Hagel responded that his questioning of the surge strategy in Iraq was not an aberration.
"I always ask the question is this going to be worth the sacrifice because there will be sacrifice," Hagel said. "Now, was it required? Was it necessary? Senator McCain has his own opinion on that, shared by others. I am not sure. I am not that certain that it was required. It doesn't mean I am right."
Other Republicans on the panel complained that Hagel failed to turn over requested copies of past speeches and financial reports they requested, and he avoided directly answering some questions seeking to force him to declare that previous positions or comments were mistaken.
Hammered during questioning by Sen. Lindsey Graham, R-South Carolina, Hagel conceded that he never should have made the comment about the Jewish lobby.
Hagel had three major preparatory sessions for Thursday's hearing, according to an administration official involved in the confirmation process. The official said Hagel chose to "take the high road" by not responding with anger to "political theater."
However, another official acknowledged Hagel had some difficulty, saying "we think he's on his way, but he didn't round the bases today. He could have. He didn't."
A plurality of Americans back Hagel's nomination to succeed Panetta.
According to a CNN/ORC International poll conducted January 14-15 and released two weeks ago, 48% of the public said the Senate should confirm Hagel, with 22% saying no and three in 10 unsure.