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Chuck Hagel is now a front-runner to become the next U.S. secretary of defense. To those who haven't followed the Senate closely in the past decade, he's probably not a household name.

Hagel is a former GOP senator and Purple-Heart-decorated Vietnam veteran, but he wouldn't necessarily be a popular pick with Republicans in Congress. Here's why.

The former Nebraska lawmaker has a long-running independent streak, one that made him an outsider within his own party by the time he left Congress in 2009, as he clashed with President Bush and Sen. John McCain of Arizona on foreign policy, most notably the Iraq War.

Hagel fiercely criticized Bush's "surge" of 30,000 additional troops to Iraq, which Bush pursued against the bipartisan Iraq Study Group's proposal of a draw-down and regional diplomacy, preferred by Hagel. When Bush instead announced that more troops would go to Iraq, Hagel co-sponsored a nonbinding resolution to oppose it, along with then-Senator Joe Biden, D-Del.

"I will do everything I can to stop the president's policy as he outlined it Wednesday night," Hagel said at the time.

"The president says, 'I don't care.' He's not accountable anymore," Hagel told Esquire in June 2007. "He's not accountable anymore, which isn't totally true. You can impeach him, and before this is over, you might see calls for his impeachment. I don't know. It depends how this goes."

Hagel's anti-Bush Iraq positions might not be his most contentious among the former Senate GOP colleagues who would vote on his confirmation. His stances on Israel and Iran could cause more of a stir.

Hagel, 66, opposes unilateral sanctions against Iran, and while his multilateralist approach fits with President Obama's, Hagel's votes against extending sanctions on Iran will likely come up.

As The Daily Beast's Eli Lake reported, Israel-backers could oppose Hagel for his Iran votes, as well as his signature on a 2009 letter urging Obama to begin talks with Hamas. Staunch Israel-backers had some negative words for Hagel for his congressional record, Lake reported.

Throughout a somewhat controversial career, Hagel has also gained stardom as a potential presidential and vice-presidential candidate, for both parties. Hagel made the short-list for Bush's VP, compiled by eventual VP Dick Cheney. But he was also asked by reporters whether he'd consider signing on as Obama's running mate in 2008, and his answer was "yes," The New Yorker recalled in a 2008 profile.

In March 2007, after rumors that he might seek the GOP presidential nomination, Hagel called a speculation-inducing news conference, only to announce that he hadn't decided whether to run.

After Hagel declined to endorse his friend McCain for president in 2008, and told The New Yorker that he likely couldn't serve in a McCain cabinet because of opposing foreign-policy views.

Former GOP colleagues will remember such political baggage, should Obama nominate him.