A White House job is a career pinnacle, but sometimes it can be too much of a good thing. "Most people last 18 months," says Erskine Bowles, a former chief of staff to President Clinton. "It's the most exhausting, most comprehensive experience in your life." This means President Obama is likely to see some high- profile exits by yearend. Below is a handicapper's guide to departures, based on interviews with top White House officials and their associates.

Larry Summers Director, National Economic Council

Probability of leaving by yearend

60%

Why he would leave

Summers, 55, has told friends that he's unhappy about his White House role, and he didn't get the coveted Fed chairmanship.

Who might replace him

Ex-Fed Governor Alan Blinder; Roger Altman, co-chairman of Evercore Partners; National Economic Council Deputy Director Jason Furman.

Why he matters

Obama sees him as the leader of his economic brain trust. He was a crucial player in helping to right the economy after the financial crisis.

Peter Orszag Director, Office of Management & Budget

Probability of leaving by yearend

50%

Why he would leave

Orszag's soon-to-be second wife lives in New York; at 41, he may desire a private-sector job—and salary. He was about to leave when Obama insisted he stay.

Who might replace him

Fiscal hawks: Robert Greenstein, director of the Center on Budget & Policy Priorities; Urban Institute President Robert Reischauer.

Why he matters

He prepared Obama's $3.8 trillion fiscal 2011 budget and was a key policy architect of the stimulus and health-care bills.

Robert Gates, Defense Secretary

Probability of leaving by yearend

40%

Why he would leave

After managing two wars since 2006, Gates, 66, is eager to retreat to his home near Seattle but may stay until the first Afghan troop withdrawals in July 2011.

Who might replace him

Obama may turn to Senator Jack Reed (D-R.I.), who could demur for family reasons, or former Senator Chuck Hagel (R-Neb.).

Why he matters

He has earned Obama's trust, led a turnaround in Iraq, and forced the Pentagon to give up speculative, costly high-tech toys.

Rahm Emanuel, White House Chief of Staff

Probability of leaving by yearend

30%

Why he would leave

He'll probably stay until mid-2011. If Chicago Mayor Richard M. Daley doesn't run in 2011—and friends say he likely will—Emanuel, 50, would jump in.

Who might replace him

Tom Daschle, ex-Senate majority leader; Emanuel aide Phil Schiliro; Tom Donilon, deputy national security advisor.

Why he matters

One of the most powerful Presidential aides in decades. Devised winning strategies to pass the stimulus plan and health-care reform.

David Axelrod, White House Senior Advisor

Probability of leaving by yearend

20%

Why he would leave

Axelrod, 55, went right from the campaign to the White House with little downtime. His wife, Susan, remained in Chicago. He is likely to stay until mid-2011.

Who might replace him

Obama would almost certainly turn to trusted insiders such as Press Secretary Robert Gibbs or former campaign manager David Plouffe.

Why he matters

Nobody else has his political savvy and personal relationship with Obama. He likely will be a top 2012 campaign strategist.