Mr Obama, who avoided a Senate confirmation battle by deciding not to nominate UN Ambassador Susan Rice as his first choice for secretary of state, went with Mr Hagel, a decorated Vietnam combat veteran, even as leading Republicans announced their opposition, though they stopped short of saying they might try to block Mr Hagel.
Seeking to soften the ground, the White House was alerting Senate Democrats that Mr Hagel's selection as the successor to Defence Secretary Leon Panetta in Mr Obama's second-term Cabinet was imminent, according to a congressional official.
Mr Obama, who has returned to Washington from his Hawaiian family holiday, was expected to nominate Mr Hagel as early as Monday. Congress is still on a break this week.
Mr Hagel, a moderate Republican, built a strong relationship with Mr Obama during their travel as senators. But the former Nebraska politician has faced withering criticism from Congress since emerging as the front-runner for the Pentagon post. In sticking with Mr Hagel, Mr Obama appears willing to take on the fight.
Mr Hagel is the second straight Obama favourite for a top national security post to face criticism from Capitol Hill even before being nominated. Ms Rice withdrew her name from consideration for secretary of state amid charges from Republican senators that she misled the public in her initial accounting of the attacks on Americans at a diplomat post in Benghazi, Libya.
If confirmed, Mr Hagel would take over a Pentagon that faces budget cuts and a scaling back of the US-led war in Afghanistan. Afghan President Hamid Karzai is expected to meet with Mr Obama in Washington this week to discuss the US presence in Afghanistan after the war formally concludes at the end of 2014.
Mr Hagel is likely to support a more rapid withdrawal of US troops from Afghanistan.
Mr Obama, in an interview that aired last week on NBC's "Meet the Press," called Mr Hagel "a patriot" who "has done extraordinary work" in the Senate and on an intelligence advisory board.