One high-profile Vietnam veteran standing by Hagel is Gen. Colin Powell, a former Joint Chiefs chairman who was secretary of state under the Republican administration of President George W. Bush.
"You can always count on him to analyze a difficult situation and take a position that reflects his best judgment," said Powell, a Republican who nonetheless had similarly endorsed Obama. "I believe that more than ever we need that kind of independent and bold leader who thinks in and out of the box."
Gen. Wesley Clark, former NATO supreme allied commander, said Hagel has earned the president's trust.
Hagel's experience in Vietnam, which earned him two Purple Hearts, gave him "the proper appreciation for what it's like on the ground, at the bottom," Clark said.
But Rep. Tom Cotton, R-Arkansas, a veteran of the wars in Iraq and Afghanistan, is not supportive.
"Our troops deserve much better than a man who voted to send them to war when it was popular and then abandoned those very troops when it was unpopular," he said.
He was referring to Hagel's 2002 vote authorizing the war in Iraq and his ultimate opposition to it.
Willingness to speak his mind stirs criticism, support
If he becomes defense secretary, Hagel will be tasked with carrying out the orders of a president who concluded the war in Iraq and plans to end U.S. involvement in Afghanistan.
Ending and avoiding war are part of what he committed his life to while in his 20s in Vietnam, Berens wrote.
"After a year of Vietnam's miserable heat, nearly constant danger, and violent campaigns like the Tet Offensive, Chuck Hagel came back to the United States ready to get on with things -- and with both a loyalty to the U.S. military and a belief he should do all he could to prevent his nation's being involved in another war."
His fierce opposition to the Iraq War went far toward creating the schism that now exists between him and the GOP establishment.
"The damage this war has done to our country will play out for years to come," he wrote in his 2008 book, "America: Our Next Chapter."
"While it is easy for nations to blunder into war, they never blunder into peace," he added.
Especially on foreign policy issues, Hagel has shown time and again his willingness to speak his mind -- even if it goes against the prevailing Republican thinking or, in the case of the 2009 Afghanistan surge, the position of Obama.
His independence has spawned critics, as well as cheers.
"He's a guy with really serious foreign policy chops and someone, frankly, who hasn't been afraid to depart from his party when he thought they were wrong," Murphy, the Democratic senator from Connecticut, told CNN.
Hagel himself said Monday that he won't hold back if he becomes defense secretary, telling Obama, "I will always give you my honest and most informed council."