WASHINGTON -- President Barack Obama’s selection of Chuck Hagel to be secretary of defense has turned the presidential nomination process into something it is traditionally not: a fight.

Obama nominated the Republican former senator from Nebraska and decorated Vietnam veteran despite strong criticism by Republicans, and even some Democrats, for his failure to support the Iraq war, his perceived lack of commitment to Israel and his remarks about a gay ambassador.

“I don’t understand why the administration is looking to pick yet another political fight instead of working with Congress to solve some of the very real problems we face as a country,” said Sen. Rob Portman, R-Ohio.

Presidents tend to avoid battles over nominations as they try to kick off their terms with cordial relations with lawmakers on Capitol Hill. But ever since Obama became the first Democrat since Franklin Roosevelt to win more than 50 percent of the popular vote twice, he has shown more of a willingness to take on a divided, unpopular Congress.

Obama did step back from one looming fight, over the likely nomination of U.N. Ambassador Susan Rice to be secretary of state. But at the end of 2012 he held multiple events blaming Congress for a lack of a deal to avert massive tax increases and spending cuts, even as the two sides were negotiating. He’s warned Congress he won’t negotiate over the need to raise the nation’s debt ceiling. And now his choice of Hagel signals another clash.

“He sees his approval rating at 55 percent and Congress at 18 percent, and he figures he can muscle his way through,” said political analyst Stuart Rothenberg.

Hagel, 66, would be the first person of enlisted rank to head the Pentagon, and the second Republican in Obama’s Cabinet after Transportation Secretary Ray LaHood. He would replace Leon Panetta, who plans to retire to his walnut farm in California.

“I think in part it’s a dare and challenge to the GOP to support one of their own,” said Sarah Binder, a congressional expert at the Brookings Institution. “If it ends up getting torpedoed by Republican votes, he can say it’s another example of people trying not to make the Senate work.”

The president rounded out his national security team Monday by picking counterterrorism adviser John Brennan to be director of the Central Intelligence Agency. Brennan, 57, would replace David Petraeus, who resigned after admitting that he had an affair.

Brennan, one of the president’s most trusted advisers, has played a central role in Obama’s drone program in Pakistan, Yemen and Somalia.

A former CIA analyst and deputy executive director of the spy agency, Brennan took himself out of consideration for the post four years ago after criticism that he had supported interrogation measures such as waterboarding and sleep deprivation that many experts consider torture. Brennan denied those allegations.

Obama’s decision to nominate polarizing figures has its risks. If Hagel does not get confirmed, or even if he squeaks through, Obama could end up looking weak – and Congress more emboldened.

The president did not acknowledge the opposition to Hagel when he made his announcement at the White House, but he mentioned his former colleague’s independent streak.

“Chuck represents the bipartisan tradition that we need more of in Washington,” he said. “In the Senate, I came to admire his courage and his judgment, his willingness to speak his mind, even if it wasn’t popular, even if it defied the conventional wisdom.”