What Emanuel leaves behind is more than a staff job. It is the most demanding and influential position in the White House—save for Obama's. The person who holds it is entrusted to shape the president's thinking, prioritize his time, manage scores of egos and issues and keep the White House focused on its goals.

Stepping into that role will be Pete Rouse, a deeply trusted senior adviser to Obama who has made much of his living as a chief of staff. Obama on Friday is expected to herald Emanuel's service, talk of unfinished business and introduce Rouse as interim White House chief of staff, likely for the rest of the year.

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The two men could not be more different in their personalities and style. Emanuel, 50, is a fast-moving, disciplined and notoriously profane manager—the once and future politician who served as an Illinois congressman and always had a longing for running for mayor of his hometown Chicago.Rouse, 64, shuns the spotlight but has quietly built up an enormous wealth of trust and relationships in Washington. Those close to him say that he provides what Obama needs—a sharp and strategic mind, a sense of continuity, a knack for troubleshooting and an ability to keep people focused on their tasks. Rouse served for years as chief of staff to then-Senate Democratic leader Tom Daschle and is known on Capitol Hill, but he won't be found schmoozing at political dinners.

The mantra in the West Wing is that no one who works for the president is irreplaceable. And yet that's how they describe Emanuel, a whirling force of ideas and energy with expertise in foreign policy, political campaigns, communications and the legislative process. Obama's aides talk of an unquestioned loss.The plan within the White House is that some of Emanuel's responsibilities will be shared among other senior officials, not just Rouse. White House officials also say it is a strength that Rouse will bring his own style to his job and that every White House expects change and needs it.