Michele Flournoy: Was under secretary of Defense for Policy from 2009-2012. Co-founded the Center for a New American Security in 2007. Was a principal deputy assistant secretary of Defense for Strategic and Threat Reduction and deputy assistant Secretary of Defense for Strategy in the 1990s.
Ashton B. Carter: Currently the deputy secretary of Defense. Previously the under secretary of Defense for Acquisition, Technology and Logistics from 2009-2011.
John Hamry: Served as deputy secretary of Defense, and undersecretary of Defense (comptroller) from 1993-1997. Currently the president and CEO of the Center for Strategic and International Studies.
James Miller: Currently the undersecretary of Defense for Policy.
Sam Nunn: Represented Georgia in the House of Representatives before being elected to the Senate in 1972. He served as chairman and ranking member of the Armed Services Committee before retiring in 1997. He is currently a director at The Coca-Cola Company and Hess Corporation.
Joe Lieberman: Currently serving his fourth and final term in the U.S. Senate. Has served as chairman or ranking member of the Homeland Security and Governmental Affairs Committee since its inception in 2004.
"That would make sense since he'll be out of government next year," Ervin says.
Michael Bloomberg: Currently mayor of New York City. Founder of financial data services firm Bloomberg LP, and among the wealthiest Americans. Bloomberg is "somebody just smart, and [can] contain the budget problem and bring some independent thinking," says O'Hanlon. He may be an unorthodox pick, but could apply the kind of business savvy skills that made Robert McNamara an effective secretary in the 1960s.
Secretary of Homeland Security
Thad Allen: Commandant of the U.S. Coast Guard from 2006-2010. Gained recognition from both political parties after his handling of hurricanes Katrina and Rita, and the Deepwater Horizon oil spill in the Gulf of Mexico. He retired in 2010 at the rank of admiral.
"He could fit on either side of the aisle," says Ervin. "He gained bipartisan appeal for his response to recent hurricanes."
Ray Kelly: Twice appointed as commissioner of the City of New York Police Department, most recently under Mayor Bloomberg. He created the country's first police counterterrorism bureau there. He was previously the senior managing director of Global Corporate Security at Bear Stearns & Co. Inc.
Bill Bratton: Was most recently chief of the Los Angeles Police Department, the third largest in the U.S. Previously chief of the NYPD and Boston Police Department.
Other potential picks:
Richard Danzig: Was senior adviser to Obama on national security issues from 2007-2008. Was secretary of the Navy from 1998-2001, and under secretary from 1993-1997. Currently board chairman at the Center for a New American Security.
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Paul D. Shinkman is a national security reporter for U.S. News & World Report. You can follow him on Twitter or reach him at firstname.lastname@example.org.
Updated 11/12/12: This article has been updated to reflect David Petraeus stepping down as CIA director.