The choice of Hagel, the first Vietnam veteran to be nominated for the post, would add a prominent Republican to Obama's Cabinet, providing some political cover for the president's plans to exit Afghanistan and make cuts to a military budget that has roughly doubled since the 2001 terrorist attacks.
But Republicans made clear Sunday that they would give Hagel a rough ride on his path to the Pentagon, questioning his support for Israel, his seriousness about the Iranian nuclear threat and his commitment to an adequate defence budget. And Obama may also face difficulties from some Democrats who are wary of negative comments Hagel made more than a decade ago about gays.
Some Obama aides had doubts about the wisdom of the choice, given Hagel's frosty relationship with members of his own party, but officials said they were confident that they could corral enough votes from both sides of the aisle to win confirmation in the Senate. White House officials confirmed Sunday that Hagel was Obama's pick for the job and said the announcement would come as early as Monday.
Rather than turning to a defence technocrat, Obama decided on an independent politician whose service in Vietnam gave him a lifelong skepticism about the commitment of US lives in overseas conflicts. Like Obama, Hagel supported the war in Afghanistan but opposed the troop surge in Iraq under President George W Bush.
Hagel, 66, served as an enlisted man in Vietnam, won two Purple Hearts and still carries bits of shrapnel in his chest. He was the co-founder of a cellular telephone company and headed an investment banking firm before being elected to the Senate in 1996. He retired in 2009 and now teaches at Georgetown University and serves as chairman of the Atlantic Council, a centrist foreign policy group.
In July 2008, during the presidential campaign, Hagel accompanied Obama, who was then in the Senate, on a six-day trip to Afghanistan, Iraq, Jordan and Kuwait. When Sen. John McCain, the Republican presidential nominee that year, asserted that Obama's motive for the trip was political, Hagel strongly defended Obama, saying in a television interview that McCain was "on thin ground" in trying to impugn Obama's patriotism.
In the Senate, Hagel voted in favor of the resolution authorizing Bush to take military action in Iraq, which passed overwhelmingly in October 2002. But he soured on the effort early, and became an advocate of the view that America had lost sight of what it was trying to accomplish and that it was overestimating its ability to change Iraqi society.
In some ways, Hagel bears a similarity to Susan E Rice, the ambassador to the United Nations and Obama's first choice for secretary of state. She withdrew her name from consideration, making way for the selection of Senator John F Kerry.
Rice, like Hagel, is a trusted Obama ally who spoke up for him during the 2008 presidential campaign and became a lightning rod for Republican attacks.
"The president wants someone whose judgment he respects on the big questions of war and peace," said Philip D. Zelikow, a senior State Department official under Bush and now a member of the President's Intelligence Advisory Board. Hagel is co-chairman of the board.
The White House is calculating that opposition to Hagel may be loud but not broad and that in end, the Senate will confirm him. Administration officials argued that voting against a Republican war hero to run the Defence Department would not be an easy vote for fellow Republicans, and they are confident that disgruntled Democrats will ultimately not deny their president his choice.
"At the end of the day, Republicans will support a decorated war hero who was their colleague for 12 years and has critical experience on veterans issues," said an administration official who requested anonymity to discuss a nomination before it was announced. "It would be hard to explain a no vote just because he bucked his party on Iraq, a war most Americans think was a disaster."
When he took office in 2009, Obama asked Robert M. Gates, the defence secretary during Bush's last two years in office, to remain in his job. Gates, a former CIA chief and deputy national security adviser, belonged to the mainstream of Republican defence orthodoxy. Hagel does not, as was evident in harsh comments from Republicans on Sunday.
Senator Lindsey Graham, R-S.C., said he personally liked Hagel but considered him "out of the mainstream of thinking on most issues regarding foreign policy."
"This is an in-your-face nomination of the president to all of us who are supportive of Israel," Graham said on the CNN program "State of the Union." "I don't know what his management experience is regarding the Pentagon - little, if any - so I think it's an incredibly controversial choice."
The Senate minority leader, Sen. Mitch McConnell, R-Ky., said on the NBC News program "Meet the Press," "I think there will be a lot of tough questions for Sen. Hagel, but he will be treated fairly by Republicans in the Senate."
For weeks, some Jewish groups sought to dissuade Obama from choosing Hagel, who once referred to advocates of Israel as "the Jewish lobby." Having failed, opponents over the weekend shifted to trying to block Hagel's confirmation.
Regional chapters of the American Jewish Committee, which has bipartisan bona fides, began circulating letters to their Democratic senators, urging them to oppose Hagel.
One such letter, obtained by The New York Times, said: "While AJC recognizes Sen. Hagel's record of service to our country and the people of Nebraska, his opinions on a range of core US national security priorities run counter to what AJC advocates and what President Obama has articulated - notably, on the efficacy of Iran sanctions, on a credible military option against Iran, on branding
Hezbollah as terrorist organization, and on the special nature of the US-Israel relationship."
Hagel and his supporters have dismissed criticism of his views on Israel, noting that he voted on several occasions to provide billions of dollars in military aid to the country. He also co-sponsored legislation that urged the international community to avoid contact with Hamas until it recognized Israel's sovereignty.
On one of the biggest security challenges the administration faces - how to slow or stop Iran's progress toward a nuclear capability - Hagel's views appear somewhat at odds with the president's. White House aides have been seeking to minimize the differences in advance of the expected nomination.
Hagel has long been an opponent of unilateral US sanctions against Iran - among other US adversaries - viewing them as counterproductive. Notably, he was one of only two senators to vote against the Iran-Libya sanctions act in 2001, arguing that it would undercut efforts to engage with Tehran.
But today, the administration describes the policy of tough sanctions against Tehran as the key to its strategy for forcing the country's leadership to reverse course on its nuclear program.
As secretary of defence, Hagel would not be directly involved in designing or enforcing those sanctions; that is the work of the Treasury and State departments. But he would be in charge of one of the other major elements of pressure: the huge buildup of US naval might, anti-missile capability and special operations in the Persian Gulf. That force is intended not only to keep the Strait of Hormuz open but also to make credible Obama's threat to use military force to prevent Iran from obtaining nuclear weapons.
"So far, Obama's big problem is that the threat to use force has not seemed credible," a former official who has worked on Iran issues with Hagel and frequently advises the administration on Iran, said last week. "The question is whether if Chuck is defence secretary, the Iranians would take seriously the thought that he is willing to use force if it comes to that."
In efforts to spur liberals to oppose the nomination, Hagel's critics have also focused on a comment he made in the late 1990s, opposing a Clinton administration ambassadorial nominee for being "openly, aggressively gay," and his past stances on gay rights issues.
Hagel has since apologized for the remark and in a recent statement said he supported the right of gays and lesbians to serve openly in the military. Gay rights groups like the Human Rights Campaign that tend to lean Democratic have not yet taken a position on Hagel's nomination.
White House officials noted that Hagel apologized for comments offending Israel backers and said opposing him because of his Iraq stance would not help war hawks since Obama, who opposed the invasion from the start, would simply pick another like-minded nominee.