WASHINGTON - Reshaping the tone and tenor of the White House, President Barack Obama on Friday replaced the colorful and caustic Rahm Emanuel with the private Pete Rouse as his chief of staff, shifting to a new phase of his presidency with a drastically different aide as trusted gatekeeper.

Emanuel's decision to quit the White House and run for Chicago mayor had been so well known that even Obama mocked the lack of suspense. But it still felt like the most important transition to date for the Obama operation, which has been fueled for nearly two years by Emanuel's demands, drive and discipline.

At an emotional farewell, Obama said, "We are all very excited for Rahm, but we're also losing an incomparable leader of our staff." Emanuel choked up as he said his goodbye.

Into the breech steps Rouse, an Obama senior adviser known around the White House as a problem-fixing, media-shy strategist and organizer. Rouse is expected to serve as interim chief for several months and may eventually get the permanent job, as the White House is in the midst of reviewing a broader shake-up.

Considered the most consuming and influential staff job in American politics, the chief of staff shapes nearly everything at the White House - how the president spends his time, how he pursues his strategies on foreign and domestic policy, how he deals with a politically deadlocked Congress and a skeptical electorate.

Distinctive, profane and combative in his approach, Emanuel was a bruising but successful manager often known simply as "Rahm." The jarring contrast between the outgoing and incoming chiefs of staff was on full display as Obama spoke of both men in the grand East Room, which was packed with staff members.

Emanuel waved to colleagues, whispered to his children in the first row and stood familiarly with his hands on hips, as if ready to get going. Rouse was quiet and stoic except for the occasional smile. He almost seemed to shy away into the background even as Obama lauded his skills and his results.

"It's fair to say that we could not have accomplished what we've accomplished without Rahm's leadership," Obama said.

The president singled out Emanuel's work on signature health care and financial reform legislation, hugged him more than once and told his audience: "I will miss him dearly."

Emanuel choked up when his turn came. He spoke of his family's immigrant background, the opportunities he has been afforded, his pride in Obama.

"I want to thank you for being the toughest leader any country could ask for," Emanuel told his boss.

In a nod to the political sensitivities of Emanuel's move, he never directly mentioned that he was running for mayor, and Obama didn't touch that, either. Emanuel, sure to be cast as an outsider by his competitors in the upcoming mayoral campaign, did not want to announce his run from Washington.

Instead, referring to the Chicago that both he and Obama call home, Emanuel said: "I'm energized by the prospect of new challenges, and eager to see what I can do to make our hometown even greater."

He is expected to formally announce his bid in the coming days, already the biggest name in a crowded race.

As for the more introverted Rouse, Obama joked: "Pete has never seen a microphone or a TV camera that he likes." Indeed, Rouse never spoke. He is not expected to become a public face of the administration or do the activities he has long avoided - appearing on the Sunday talk shows or attending political dinners.

He will move into Emanuel's giant corner office, though, and command the job of keeping the staff focused on Obama's directives. A veteran of Capitol Hill politics, Rouse offers Obama continuity and comfort, having served as his Senate chief of staff, campaign adviser and resident White House fixer.

Valerie Jarrett, one of Obama's senior advisers, put it this way: "When I walk into a room and see Pete, I feel better. And everybody else does, too."

Still, within the building, the confidence in Rouse came packaged with a sense that Obama had lost a leader.

Emanuel's biting words could get him in trouble. And his preference for results over ideology made him a sometimes hated figure for Obama's liberal base of supporters, especially when it became known that Emanuel was pushing a piecemeal approach on health care reform. (Obama trumped him on that.)

He offered, though, a force of personality and range of political experiences that worked for Obama. He swore and yelled. His stamp was everywhere.

"All of that will be missed," said David Axelrod, a top Obama adviser. "There's a talented group of people here who are ultimately motivated by the president and more than capable of carrying on. It may be that portfolios will change and be expanded because Rahm took up so much real estate. But I think we'll be fine."

Axelrod himself is expected to leave the White House next year to help shape Obama's re-election bid. Obama has already seen key departures among his economic and national security teams and is likely to see more, including Cabinet changes. It is a part of the rhythm of the White House, a grinding place to work.

Emanuel has a huge challenge ahead in the mayor's race, where other candidates have hardly been scared away by his intentions. They are all going for the seat long held by Chicago Mayor Richard M. Daley, who announced in early September that he would not seek a seventh term.

Ever the political operative, Emanuel got a reminder of his own ways earlier Friday.

Before a smiling collection of senior staff members in the Roosevelt Room, economic adviser Austan Goolsbee gave Emanuel a dead fish wrapped in Chicago newspapers. An angry Emanuel had once famously done the same thing to a Democratic pollster with whom he was less than pleased.