He has also faced opposition from gay-rights groups, strong supporters of Obama's election campaign, for a comment Hagel made in 1998 in which he questioned whether a nominee for an ambassadorship was suitable because he was "openly, aggressively gay." He apologized for that remark in December.

If nominated and confirmed, Hagel would be handed the challenge of closing the final chapter on the U.S.-led war in Afghanistan and overseeing the continued footprint of a smaller U.S. training force there.

Hagel has been critical of U.S. policy in Afghanistan. In 2009 he opposed Obama's 30,000-troop surge, telling the National Journal, "I'm not sure we know what the hell we are doing in Afghanistan."

"It's not sustainable at all; I think we're marking time as we slaughter more young people," he said. Later he called for the United States to stop its "nation-building" there.

He would also have to tackle the Pentagon's budget. Hagel has said in the past that be believes the Pentagon's budget is too big.

"The Defense Department, I think in many ways, has been bloated," Hagel said in a September 2011 interview with the Financial Times. "So I think the Pentagon needs to be pared down."