Citing those threats, Levin said the committee could not allow further delays to Hagel's nomination.
"We need a secretary of defense," Levin said.
Democrats highlighted Hagel's military background and the unique perspective he would bring to the Pentagon while arguing for giving a second-term president his Cabinet choices.
Sen. Tim Kaine, D-Va., said he wants a secretary of defense worrying about the men and women of the military, and he was certain that would be Hagel's overriding concern.
Republicans contended that Hagel was out of the mainstream on numerous critical issues, from threats in the Middle East to Iran's nuclear ambitions. His halting testimony at his confirmation hearing last month also troubled senators.
"The next secretary of defense will deal with a world on fire, and the testimony of Sen. Hagel was not reassuring," said Sen. Lindsey Graham, R-S.C. McCain said Hagel's performance was the worst of any nominee to testify before the committee.
Hagel's allies acknowledged his struggles, with Sen. Jeanne Shaheen, D-N.H., saying she wished he were feistier and Sen. Angus King, I-Maine, saying Hagel should have been more forceful. But they argued that should not disqualify the nominee.
All 55 Senate Democrats are expected to vote for Hagel, and two Republicans - Sens. Thad Cochran of Mississippi and Mike Johanns of Nebraska - have announced their support for the nominee. Although McCain and several other Senate Republicans oppose the choice, they have said they won't take the unprecedented step of delaying a vote for defense secretary.
Privately, Republicans say a drawn-out filibuster would be unsuccessful.
More than a dozen Republicans have said they will oppose their former colleague, and several others have indicated they are likely to vote no. Sen. Kelly Ayotte, R-N.H., a member of the Armed Services Committee, said Tuesday she would vote against the nominee, citing his performance at his confirmation hearing.
Hagel seemed ill-prepared under withering cross-examination from committee Republicans in nearly eight hours of testimony on Jan. 31. He was repeatedly pressed about past statements and votes on Israel, Iran and nuclear weapons, with GOP lawmakers suggesting he wasn't sufficiently supportive of Israel or anti-Iran.
In the memo circulated Tuesday, Republicans sought to draw comparisons to the 2005 fight over President George W. Bush's nomination of John Bolton to be ambassador to the United Nations, when Democrats tried to stop the nomination. The position is not Cabinet-level, however.
Faced with a Democratic filibuster, Bush circumvented the Senate and made Bolton a recess appointment.
One senator, Republican David Vitter of Louisiana, did not vote in the committee Tuesday.