The chatter about when Defense Secretary Robert Gates will leave the Obama administration picked up again Thursday when the Washington Times published an article speculating that he will retire in April, 2011, after unveiling the fiscal 2012 budget.

Predicting Gates's departure is one of the most popular parlor games in Washington. Gates used to carry a key chain with him at all times that ticked down the days until Jan. 20, 2009, when he had assumed he would hand over the reins of the military and return to private life. But with that date long gone, nobody really knows when Gates will leave - including, it seems, the secretary himself. He's taken on some major projects that he wants to see through, and the White House is imploring him to stay because they depend on him to oversee two wars abroad and defend the administration's policies at home.

But eventually, Gates will step down. Based on interviews with officials, staffers, and experts, here is the current short list of potential successors, along with an assessment of the strengths and shortcomings they would bring to one of the most challenging and important jobs in the world. Here they are, in no particular order:

The brains of the Pentagon: Michèle Flournoy

Under secretary of defense for policy

Flournoy is the odds-on favorite for the job. The third-ranking official at the Pentagon now, she represents the Defense Department at deputies meetings at the White House, has led the Pentagon's major policy initiatives, such as the Quadrennial Defense Review, and is well respected across the board, making her a logical choice to be the first woman ever to take the top job at DOD. Working against her is her relatively low political profile in Washington and a lack of the kind of international star power that is often needed to deal with powerful foreign leaders. Some say privately that she needs some more experience before reaching cabinet-level status, although few doubt she could do the work ably.

The rock star: Hillary Clinton

Secretary of State

Speculation about Clinton moving from State to DOD began this spring, based more on the media's fascination about the idea than any hard evidence either she or the White House is contemplating such a move. The possible benefits are obvious: The administration doesn't have anybody more famous on its roster besides Obama, she's knowledgeable on military issues having spent years dealing with them in Congress, and the uniformed leadership respects and trusts her. But Clinton is often said to be considering retirement after leaving Foggy Bottom. Her able stewardship of the State Department has alleviated concerns about her management abilities that resulted from her presidential campaign.

The establishment choice: John Hamre

President of the Center for Strategic and International Studies

Hamre, a former deputy secretary of defense and the current chairman of the Defense Policy Board, is a nonpartisan expert, by all accounts brilliant, whose decades of involvement in the defense establishment have put him on the short list for SecDef multiple times. As the leader of a top think tank, he has stature in Washington, and is close enough to industry to keep the trains running on time while being independent enough to take on the Beltway bandits when necessary. The question about Hamre is whether he would take the job if offered. He took himself out of consideration in 2009 out of loyalty to his CSIS staff. If not Hamre himself, Obama could look for a similar figure, a senior policy leader who shares the quiet confidence and steady demeanor that have made Gates such a success.

The warrior legislator: Jack Reed

Rhode Island senator

On paper, Reed is the perfect choice. A Democratic senator who knows more about the military than almost anyone in Congress, he's got the substantive chops to do the job. As a former Army ranger, he could win over the services as well. But going from running a 50-person Senate office to running a 4 million-person organization is quite a leap and nobody can know if Reed has the managerial skills to pull it off. He's also privately communicated to those close to him that he's simply not interested. He likes being a senator and representing the people of Rhode Island, so speculation about him may be a nonstarter.

Always the bridesmaid: Chuck Hagel

Former Nebraska senator

The Obama administration has been trying to give Hagel a prime posting for a while. He's reported to have turned down several sweet offers, including ambassador to China and director of national intelligence. He does serve as a co-chair of Obama's Foreign Intelligence Advisory Board. In Hagel's favor, he's a Republican who won't bend to the GOP talking points of the moment and he's high-profile enough to fill the chair. The fear is that his independence and his penchant for veering off message in a Biden-esque way make him too risky to entrust with such a prominent perch. Moreover, if Obama chooses another Republican to lead DOD, many will begin to question whether the Democratic national- security bench is too thin or if Obama is wary of putting someone in his own party in charge of the wars.  

The Sacred Cow: Sam Nunn

Former Georgia senator

Nunn, like Hamre, is always on the short list and is said to have lobbied hard for himself the last time this job was up for grabs. Nunn is a living legend in national-security circles; his work on nonproliferation and loose nuclear material was taken up by Obama himself when the then-future president joined the Senate. But Nunn is 72 years old, and his window of opportunity may simply have passed. The same could be said of his 78-year-old partner on nonproliferation issues, sitting Indiana Sen. Richard Lugar.

Passed over but not forgotten: Richard Danzig

Former Navy secretary

Danzig, one of Obama closest advisors during the 2008 presidential campaign, was widely speculated to become defense secretary in 2009. As the story goes, when Obama ultimately decided he wanted Gates to stay on, Gates requested that Danzig not be named the Pentagon's No. 2 because he didn't want to cause confusion by having a "secretary-in-waiting" right next door. Now, more than a year later, Danzig remains on the outside looking in as chairman of the board of the Center for a New American Security, but his ties to Obama persist and we're told privately that he's been promised a cabinet-level position when the big turnover comes. That doesn't mean defense, but Danzig is rumored to have turned down another cabinet-level post before, so it's not clear that offering him Homeland Security or the U.N. job would suffice. On the merits, Danzig is more than qualified, but many worry that his extreme intellectualism and iconoclasm are just not conformist enough for a White House that likes people who stay within the lines. His legendary Winnie-the-pooh speech is often cited as an example of this phenomenon.

The dark horse: David Petraeus

ISAF commander

Yes, that's right: Petraeus could be a candidate for defense secretary. He's the man Obama and the world depend on most regarding the wars in Afghanistan and Iraq. He's a brilliant scholar, a master tactician, and a seasoned diplomat. He would be confirmed by the Senate unanimously and would be able to hit the ground running with the full support of the military, sharing Gates's vision of where the Pentagon needs to go. Sure, his selection would raise serious questions about civilian control of the military, and he would have to take off his uniform before being eligible for the job. But if there's any man who could pull that off, it's Petraeus. Plus, Obama could continue his practice of taking potential challengers out of contention for November 2012 (for the record, Petraeus has repeatedly and emphatically denied holding any presidential ambitions). Congress would also have to change the law that requires military men to wait 7 years before heading up the Pentagon.

Honorable mention: Leon Panetta

CIA Director

Panetta is getting a reputation for being able to navigate the national security bureacracy with skill and savvy. He has experience in the White House and on Capitol Hill, where as House Budget Chairman he played a role in what evenutally became the budget surpluses of the late 1990s. He has successfully defended the power of the CIA while keeping Congress on his side.  If selected, he would be the second former CIA director in a row to take over the Defense Department.