"Chuck knows that war is not an abstraction," Obama said. "He understands that sending young Americans to fight and bleed in the dirt and mud, that's something we only do when it's absolutely necessary."
Critics of Hagel, co-chairman of the president's Intelligence Advisory Board and Vietnam veteran, have taken issue with his opposition to some Iranian sanctions and positions regarding U.S.-Israeli relations.
Former U.s. Secretary of State Madeleine Albright joined a prestigious list of former Cabinet officials, ranging from former National Security Adviser Zbigniew Brzezinski to Panetta's predecessor Robert Gates, in endorsing Hagel's nomination.
"His approach to national security and debates about the use of American power is marked by a disciplined habit of thoughtfulness that is sorely needed and these qualities will serve him well as secretary of defense at a time when the United States must address a range of international issues that are unprecedented in scope," they write.
Sen. John McCain, R-Ariz., ranking member of the Senate Armed Service Committee, said he had "serious concerns" about Hagel's position on a "range of critical national security issues."
In early January, Sen. John Cornyn, R-Texas, wrote for CNN that he was frustrating that Hagel expressed a need to engage diplomatically with Hamas, which runs Gaza yet is labeled as a terrorist organization by some governments, including the United States.
The Senate is expected to take up Hagel's nomination Thursday.
Notable deaths of 2014 [PHOTOS]