At one point, Hagel mistakenly said the Obama policy toward Iran is "containment" even though the former senator has said all options, including military force, should be on the table. He was handed a note and corrected himself.
Hagel was the lone witness in a jam-packed hearing room at a session that could be crucial in determining whether he will win Senate confirmation and join Obama's second-term national security team. He spoke out forcefully for a strong military while trying to explain 12 years of Senate votes and numerous statements.
"No one individual vote, no one individual quote or no one individual statement defines me, my beliefs, or my record," Hagel said in his opening statement. "My overall worldview has never changed: that America has and must maintain the strongest military in the world, that we must lead the international community to confront threats and challenges together, and that we must use all tools of American power to protect our citizens and our interests."
Hagel, 66, would be the lone Republican in Obama's Cabinet, the first Vietnam veteran to be defense secretary and the first enlisted man to take the post. That last point was highlighted by Levin.
"It would be a positive message for our soldiers, sailors, airmen and Marines in harm's way around the world to know that one of their own holds the highest office in the Department of Defense and that he has their backs," Levin said.
Two former committee chairmen — Democrat Sam Nunn and Republican John Warner — introduced the nominee.
"War for Chuck Hagel is not an abstraction," Nunn said.
Hagel has the announced backing of about a dozen Democrats and the tacit support of dozens more who are unlikely to embarrass the president by defeating his Cabinet pick. One Republican — Sen. Thad Cochran of Mississippi — has said he will vote for his former colleague.
Eight Republicans, including four members of the Armed Services panel, have said they will oppose Hagel's nomination. Dan Coats of Indiana, who served with Hagel in the Senate, and Marco Rubio of Florida announced their opposition on Thursday.
Democrats hold a 55-45 advantage in the Senate and a 14-12 edge on the committee.
Republicans repeatedly questioned Hagel about a May 2012 study that he co-authored by the advocacy group Global Zero that called for an 80 percent reduction of U.S. nuclear weapons and the eventual elimination of all the world's nuclear arms.
The group argued that with the Cold War over, the United States can reduce its total nuclear arsenal to 900 without sacrificing security. Currently, the U.S. and Russia have about 5,000 each, either deployed or in reserve. Both countries are on track to reduce their deployed strategic warheads to 1,550 by 2018, the number set in the New START treaty that the Senate ratified in December 2010.
Fischer, the Nebraska Republican, quoted from the Global Zero report and expressed strong misgivings.
"Many of my colleagues are concerned you've changed your views. My concerns are that you haven't changed your views. You continue to hold extreme views, far to the left even of this administration," said Fischer.
In last year's Senate race, Hagel endorsed Fischer's Democratic rival, former Sen. Bob Kerrey, who also is a Vietnam combat veteran.
Hagel insisted that the report was merely illustrative and said it wasn't realistic to consider unilateral nuclear weapons reductions.
Associated Press writer Erica Werner contributed to this report.