Michele Flournoy conducts a press conference at the Pentagon in 2010, while she served as undersecretary of defense for policy. She is considered a leading candidate to become the next Pentagon chief. (Photo by Cherie Cullen/Defense Department)

Defense Secretary Chuck Hagel is stepping down under pressure. The news emerged Monday morning, just a couple hours before an expected announcement by President Obama. It isn’t a complete shock — Hagel has long been derided as indecisive — but it amounts to one of the most significant administration shake-ups in Obama’s time in the White House.

Who could replace Hagel? It’s worth noting that any successor will likely have to get through a confirmation process in which Sen. John McCain (R.-Ariz.) leads the Senate Armed Services Committee. He’s a vocal critic of the Obama administration, and pounded Hagel with difficult questions during his confirmation process in January 2013. Hagel survived — but just barely.

A look at Hagel’s possible replacements:

Michèle Flournoy
One of the names immediately pondered is Michèle Flournoy. She served as the under secretary of defense for policy from February 2009 to February 2012, while Robert Gates and Leon Panetta ran the Pentagon. She said she need to rebalance her life when she stepped down, but has remained active in Washington. But in the past, Flournoy has been seen by some in the White House as too uncritically supportive of uniformed military’s perspective, suggesting that her appointment might set Obama up for the kind of clashes he had with some military leaders over Afghanistan in his first term.

Flournoy is currently the chief executive officer at the Center for a New American Security, a non-partisan think tank that the Obama administration is believed to have relied upon in developing national security policy. She co-founded CNAS in 2007, and served as its president until 2009, when she took her under secretary job.

Robert Work
Robert Work is currently the deputy defense secretary, and has previously served as undersecretary of the Navy. Work, a retired Marine colonel, also served as the CEO of CNAS before the Senate confirmed him in his present position in April.

Work has a reputation for being a blunt speaker, and as the Pentagon’s No. 2 leader, has had a major role in examining the Defense Department’s budget and a variety of crises in recent months. For example, he currently is chairman of the Nuclear Deterrent Enterprise Review Group, which is assessing how the Pentagon should manage its aging nuclear weapons arsenal in light of several recent scandals.

Ashton Carter
Ashton Carter served as the Pentagon’s No. 2 official from October 2011 until December 2013, and stepped down after being bypassed in favor of Hagel for the job.

Like Work, he oversaw efforts to reduce the Pentagon’s budget, and has been a fixture in the national security world for years. He joined the Obama administration in 2009 as the Pentagon’s top weapons buyer, and moved up into the No. 2 job two years later.

Carter, a former Rhodes Scholar and Harvard professor, is seen as an effective communicator who understands the ins-and-outs of the Defense Department’s unwieldy bureaucracy. But he has not been an influential political player, which could hurt his chances for being nominated for the top Pentagon job.

Other options
Other names have been floated, including Sen. Jack Reed (D-R.I.) A spokesman for Reed said Monday morning that the senator is not interested in the job.

“Senator Reed loves his job and wants to continue serving the people of Rhode Island in the United States Senate,” said a spokesman, Chip Unruh. “He has made it very clear that he does not wish to be considered for Secretary of Defense or any other cabinet position. He just asked the people of Rhode Island to hire him for another six year term and plans on honoring that commitment.”

Navy Secretary Ray Mabus was discussed as a possible option in 2011, before Panetta took the job. Army Secretary John McHugh also has been among the Pentagon’s administrators for years, and is a former Republican congressman from New York.

Recent senior general officers are not typically considered because of federal law, which prohibits anyone from becoming secretary of defense until seven years have passed their last day in uniform. That would eliminate high-profile generals like James Mattis and Stanley McChrystal, although they also reportedly clashed with the Obama administration.

There is a notable exception to that rule: President Truman selected Gen. George Marshall to become secretary of defense in 1950, five years after he retired as a military officer. Marshall was an exception in several ways: He also served as secretary of state from 1947 to 1949. Marshall becoming secretary of defense required a waiver from Congress.

Missy Ryan and Ed O’Keefe contributed to this report.

President Obama announced the resignation of Defense Secretary Chuck Hagel at a joint news conference on Monday. Hagel will continue as defense secretary until the president nominates a successor, who must also be confirmed by the Senate. (Associated Press)

This post has been updated.

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