In addition to calling for talks with the Middle Eastern nation, which openly antagonizes Israel, Hagel has spoken out against certain sanctions -- a cornerstone of Obama's foreign policy aimed at pressuring Tehran over its nuclear program.
The New York Times notes that Hagel 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."
In a September editorial in the Washington Post, Hagel -- writing with several others, including retired Gen. Anthony Zinni and former Rep. Lee Hamilton -- said that he thinks "all options (should be) on the table" regarding Iran, a stance that aligns him with Obama, as well as many Republicans.
GOP senator: Hagel would be 'antagonistic' toward Israel
But the most charged subject is likely Israel, with Graham arguing -- if confirmed -- Hagel "would be the most antagonistic secretary of defense" toward Israel "in our nation's history."
Hagel has supported Israel entering negotiations with Hamas, the Islamist movement that oversees parts of the Palestinian territories and is considered a terrorist organization by the United States, while also insisting that Hamas end terrorism and accept Israel's right to exist. He also was one of 12 senators who refused to sign a letter to the European Union trying to designate Hezbollah as a terrorist organization."
In 2007, Hagel joined two other senators in introducing a resolution in June 2007, pushing for a two-state solution to the Israeli-Palestinian conflict.
In an interview that year, Hagel said a "Jewish lobby intimidated lawmakers" -- sparking sharp criticism from some Jewish organizations and across the political spectrum. Graham has blasted what he calls "an in-your-face nomination by the president to all of us who are supportive" of Israel.
But others have rallied around him, including his friend Rabbi Aryeh Azriel of Temple Israel in Omaha, Nebraska.
Azriel, who grew up in Israel and says he's personally open for talks with Hamas and Hezbollah, praised Hagel as "definitely a friend of Israel. He is independent, has wonderful, fresh ideas to try to reengage the discussion about the Middle East."
Hagel told the Journal Star there is "not one shred of evidence that I'm anti-Israeli, not one (Senate) vote that matters that hurt Israel," claiming his critics have "completely distorted" his record.
He emphasized his "unequivocal, total support for Israel" and support for tough international economic sanctions against Iran, the paper reported.
Such fiery rhetoric has prompted presidential nominees in the past to pull out. But the bumpy road ahead is unlikely to faze Hagel.
"Chuck Hagel is not afraid of challenge -- or risk," his biographer, Charlyne Berens, wrote in 2006.
Praise from Powell, criticism from Iraq vet
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.