Hagel, even before being nominated, has faced tough criticism from congressional Republicans who say the former GOP senator is anti-Israel and soft on Iran. And Brennan, a 25-year CIA veteran, withdrew from consideration for the spy agency's top job in 2008 amid questions about his connection to enhanced interrogation techniques during the George W. Bush administration.
Administration officials, speaking on condition of anonymity, say Obama will announce both nominations at a White House event Monday afternoon. Along with secretary of state nominee Sen. John Kerry, D-Mass., Hagel and Brennan would play key roles implementing and shaping Obama's national security priorities in a second term. All three men must be confirmed by the Senate.
In nominating Hagel, Obama signaled he is willing to take on a tough confirmation fight. Once Hagel emerged as Obama's likely nominee, GOP lawmakers began sharply questioning his commitment to Israel and his willingness to take a hard line with Iran over its disputed nuclear program.
Hagel, a 66-year-old moderate Nebraska Republican, has criticized discussion of a military strike by either the U.S. or Israel against Iran. He also irritated some Israel backers with his reference to the "Jewish lobby" in the United States. And he has backed efforts to bring Iran to the table for future peace talks in Afghanistan.
White House officials say Hagel's positions on Israel and Iran have been misrepresented. They cite his Senate votes for billions in military assistance to Israel and his support for multilateral sanctions on Tehran.
Ben Rhodes, Obama's deputy national security adviser, said Hagel will be "completely in line with the president" on both issues.
"The president has a record of unprecedented security cooperation with Israel, and that's going to continue no matter who the defense secretary is," Rhodes said.
Hagel has also been criticized by some Democrats for saying in 1998 that a nominee for an ambassador post was "openly, aggressively gay." He has since apologized for those comments.
Hagel is the second straight Obama favorite for a top national security post to face criticism from Capitol Hill even before being nominated. United Nations Ambassador Susan Rice withdrew her name from consideration for secretary of state amid charges from GOP senators that she misled the public in her initial accounting of the attacks on Americans at a diplomatic post in Benghazi, Libya.
Obama returned to the White House on Sunday after a truncated family holiday in Hawaii. His week will also include a visit from Afghan President Hamid Karzai. And there's a bruising fight with Congress over spending and the federal deficit on the horizon, just days after Obama and Congress averted the fiscal cliff with a last-minute deal over the New Year's holiday. But rounding out his national security team in his first project.
Both Hagel and Brennan have close relationships with Obama, who values loyalty in his inner circle. Brennan, as the president's top counterterrorism adviser, was deeply involved in the planning of the 2011 raid that killed Sept. 11 mastermind Osama bin Laden. And he has led administration efforts to quell the growth of terror organizations in Yemen and elsewhere in the Middle East and North Africa.
Brennan, 57, spent a quarter-century at the CIA. He served as station chief in Saudi Arabia and in a variety of posts, including deputy executive director, during the Bush administration.
His tenure at the agency during Bush's presidency drew criticism from liberals when Obama considered naming him CIA director after the 2008 election. Brennan denied being involved in the Bush administration's enhanced interrogation techniques, but still withdrew his name from consideration.
In a letter to Obama at the time, Brennan said he was "a strong opponent of many of the policies of the Bush administration, such as the preemptive war in Iraq and coercive interrogation tactics, to include waterboarding." Many people consider waterboarding and other harsh interrogation methods to be torture.
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White House officials say they don't expect Brennan to face similar trouble this time around given his four years of service in the Obama administration.
"The issue has been removed from the debate because the president and John Brennan, as his top counterterrorism adviser, brought those techniques to an end," Rhodes said.
However, Brennan's nomination will likely put a spotlight on the administration's controversial drone program. Brennan was the first Obama administration official to publicly acknowledge the highly secretive targeted killing operations.
Brennan has defended the legality of the overseas drone operations and has said they protect American lives and prevent potential terror attacks.
If confirmed, Brennan will succeed David Petraeus, who resigned in November after admitting to an affair with his biographer.
Deputy CIA director Michael Morell has been serving as the agency's acting director since Petraeus resigned and was considered by Obama for the top job. Rhodes said Morell will attend Monday's White House event and is expected to stay at the CIA.
Hagel would replace retiring Pentagon chief Leon Panetta at a time when the Defense Department is facing potentially deep budget cuts. Hagel would also be tasked with overseeing the military drawdown in Afghanistan, where the U.S.-led war is scheduled to end in two years.
Hagel is likely to support a more rapid withdrawal of U.S. troops from Afghanistan than some military generals.
Sen. Mitch McConnell, the top Senate Republican, said Sunday he was reserving judgment on whether to support Hagel but predicted the former senator would face serious questions.
Any nominee must have "a full understanding of our close relationship with out Israeli allies, the Iranian threat and the importance of having a robust military," McConnell said on ABC's "This Week."
The second-ranking Senate Republican, John Cornyn of Texas, said in a statement that making Hagel defense secretary would be "the worst possible message we could send to our friend Israel and the rest of our allies in the Middle East."
Despite the criticism, no Republican lawmakers have threatened to try to block Hagel's nomination.
Monday's nominations leave Obama without a woman in line for a top administration post, a fact that has irked some Democratic women. The president will soon name a new treasury secretary, but current White House chief of staff Jack Lew is the front-runner for the post.
Press; By JULIE PACE]
Associated Press writers
Robert Burns, Donna Cassata and Matthew Daly contributed to this
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