Since he left the Senate, Hagel has been a big critic of his own party. He told the Financial Times in 2011 that he was "disgusted" by the "irresponsible actions" of Republicans during the debt-ceiling debate.

In 2012 he endorsed a Democrat candidate for Senate from Nebraska - former Senator Bob Kerrey - instead of Republican Deb Fischer, who won.

Hagel would not be the first Republican to serve Obama as Pentagon chief. Bob Gates, Obama's first defense secretary, was a holdover from the years of Republican President Bush.

AS A SENATOR, CLASHED WITH OTHER REPUBLICANS

While he was in the Senate as a senior member of the Foreign Relations, Banking, and Intelligence Committees, Hagel often clashed with his party's leaders on foreign and defense policy.

He co-sponsored legislation to ease US trade restrictions with Cuba, and voted against trade sanctions on Iran and Libya.

In 2002 Hagel said the US should try to improve relations with the countries Bush had branded an "axis of evil" - Iraq, Iran, and North Korea.

The same year, when Hagel expressed doubts about the Bush administration's buildup to war in Iraq, the conservative Weekly Standard magazine branded him part of an "axis of appeasement." But Hagel did vote to give the president the authority to carry out the March 2003 invasion.

Later Hagel said he regretted that vote and became a persistent critic of the conflict. In January 2007, he was the only Republican on the Senate Foreign Relations Committee to support a non binding measure that criticized Bush's decision to send more troops to Iraq.

Hagel told senators they must take a stand on "the most divisive issue in the country since Vietnam," a war in which he fought, but later decided was wrong. His stance put him at odds with his fellow Republican maverick, McCain, and Hagel was pilloried by other Republicans.

Former Vice-President Dick Cheney told Newsweek: "I believe firmly in Ronald Reagan's 11th Commandment: thou shalt not speak ill of a fellow Republican. But it's very hard sometimes to adhere to that where Chuck Hagel is involved."

In 2008, Hagel did not make a public endorsement in the presidential race, but his wife Lilibet endorsed Obama and sat with Obama's wife Michelle during the last presidential debate.

Hagel skipped the 2008 Republican convention to travel to Central and South America. Then he further irked Republicans by telling the Omaha World-Herald newspaper that it was a "stretch" to say McCain's running mate Sarah Palin would be qualified to be president.

He was once considered a contender for the 2008 presidency himself, and there was speculation he would join New York Mayor Michael Bloomberg on an independent ticket. Instead he said in September 2007 that he was dropping out of politics and retiring from the Senate when his term ended in 2008.

Born in 1946, Hagel grew up in Nebraska as the oldest of four boys, and made a fortune by launching a cellphone company in the 1980s. His father was also a military man, a World War II veteran who died of a heart attack when Chuck was 16.

Hagel and his younger brother Tom volunteered for Vietnam, and Hagel saved Tom's life there by pulling him out of a burning vehicle.