Although no final decision has been made because of family considerations, ABC News has learned that White House officials are preparing for Chief of Staff Rahm Emanuel to announce on Friday -- as Congress adjourns for recess -- that he is leaving his post to explore a run for mayor of Chicago.
White House officials expect that President Obama will also name an interim chief of staff, perhaps senior adviser Pete Rouse, at the announcement.
Sources close to Emanuel cautioned that he has yet to pull that last trigger on the decision.
Emanuel's likely departure is not a surprise; his mayoral aspirations are well known.
Longtime Chicago Mayor Richard M. Daley's announcement earlier this month that he would not seek reelection created the opportunity that Emanuel has long been seeking.
The former Chicagoan has never been coy about his desire to head the proverbial City With Shoulders.
"One day I would like to run for mayor of the city of Chicago. ... That's always been an aspiration of mine, even when I was in the House of Representatives," Emanuel said in April to Bloomberg's Charlie Rose.
With Daley's announced exit, ambitious Chicago Democrats quickly began angling to replace him, creating pressure for Emanuel to make a decision on his White House position soon, even coming from the president himself.
"I think that Rahm will have to make a decision quickly, because running for mayor of Chicago is a serious enterprise," President Obama said Monday on NBC's "Today Show." "He hasn't told me yet. But as soon as he does, I'm sure that we'll announce it."
The president has been clear that Emanuel has his blessing when the job opened, aides calling it an "unbelievably attractive opportunity" for anyone that the president would support.
The president has said that Emanuel would make an "excellent" and "terrific" mayor, but would not answer if he would offer up a presidential endorsement yet.
Emanuel has to declare his intent to enter the race by Nov. 22, the filing deadline in Chicago. Candidates need to collect 12,500 signatures by that day to qualify for a Feb. 22, 2011 Democratic primary.
Emanuel came to the White House with a reputation and political identity in-and-of himself. The feisty four-term former congressman from Illinois is a veteran of the Clinton administration and a close Obama political ally from the president's early days in Chicago.
Emanuel brought experience and knowledge of Capitol Hill to the White House when he started in January 2009, as well as a sense of loyalty to the president, who had been in his inner circle for years.
Sources close to Emanuel say that the decision on whether to run for mayor has not been easy decision for him, because family considerations -- uprooting his wife Amy and three children for the second time in as many years -- weighed heavily on him.
Emanuel delayed his original decision to give up his seat in Congress and run the White House staff. At the time, Emanuel held aspirations of being the first Jewish speaker of the House, an accomplishment that now seems more likely to be achieved by Rep. Eric Cantor, R-Va.
After much personal prodding by then President-elect Obama, Emanuel decided to join the administration, where he remained for 21 months.
During his time in the West Wing, Emanuel helped shepherd the health care act through Congress, pass the American Recovery and Reinvestment Act, and pass financial regulation.
While others in the White House, like senior adviser David Axelrod, are the voice of the liberal conscience, Emanuel was often the brass tacks pragmatist. Many progressives and liberals held him responsible for not pushing harder -- or at all -- for a public option in the health care legislation, or for criminal trials for Guantanamo defendants linked to the 9/11 attacks.
Known for his brash personality, Emanuel began each day before the sun, often by swimming a mile, and was perhaps the hardest worker in the White House, a requirement for the all-consuming job of chief of staff.
White House press secretary Robert Gibbs this month joked about one of Emanuel's favorite phrases, summing up his work ethic during his time at the White House.
"As Rahm sometimes jokes," Gibbs recalled, "if it's Saturday, there's just two more full workdays until Monday."
Emanuel's likely departure comes at a critical time for the White House and for Obama. Just five weeks before the crucial midterm elections when Democrats in races across the country face possible defeat, the administration is also balancing tough poll numbers.
The most recent ABC/Washington Post poll from early September shows the president's approval ratings at 46 percent, with a 52 percent disapproval rate.
A new incoming chief of staff could bring in energy and fresh ideas, and help the White House retool operations, as the administration picks up the pieces of what could be a difficult election night for Democrats.
Obama is known for his loyalty, maintaining a tight inner circle, and is not liking to bring outsiders into his administration, so it is likely that the next pick for chief of staff would come from someone with experience inside the administration and a relationship with the president.
Vice President Biden's chief of staff Ron Klain, Deputy National Security Advisor Tom Donilon and White House deputy chief of staff Jim Messina are all said to be on the list of candidates from within the president's inner circle and already working at 1600 Pennsylvania Avenue.
Yet the choice may also be made to appoint someone from outside the administration to capitalize on some fresh blood and new ideas within the administration.
Former Senator Majority Leader Tom Daschle and Obama's 2008 campaign manager, David Plouffe, are both said to be on the list of names to be considered from outside the administration.
Emanuel's departure is just of many departures the White House has faced recently, including many from the president's economic team.
Last week the White House announced that National Economic Council director Larry Summers is expected to leave the administration after the mid-term elections.
Earlier this month Christina Romer resigned as the chairwoman of the President's Council of Economic Advisors.
And Office of Management and Budget director Peter Orzsag resigned over the summer as well.
The White House has been working hard to downplay the departures.
"I think there's no doubt that there will be people that return to their lives and their families," Gibbs said of the typical turnover that occurs within presidential administrations. "But we've got a while before that. We've got at least two months before this election -- or about two months before this election before we get to a lot of those decisions."
Early on in the administration Ellen Moran, the first communications director; Anita Dunn, the second communications director; and Desiree Rogers, the social secretary, also resigned.