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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.