Updated Oct. 1, 12:47 p.m.
President Barack Obama formally announced Friday what everybody in Washington already knew: his chief of staff, Rahm Emanuel, is leaving the West Wing to run for mayor of Chicago.

“Welcome to the least suspenseful announcement of all time,” Obama joked to reporters during Emanuel’s send-off in the East Room.

Obama named senior adviser Peter Rouse as Emanuel’s interim replacement. Rouse was Obama’s chief of staff when he was a Senator; prior to that, he was chief of staff to former Senate Majority Leader Tom Daschle (S.D.) for nearly 20 years. Both stood alongside Obama during the announcement.

“This is a bittersweet day here in the White House,” the president said. “On the one hand, we are all very excited for Rahm as he takes on a new challenge. … But we’re also losing an incomparable leader of our staff and one who we are going to miss very much.”

He praised Emanuel’s “unmatched level of energy and enthusiasm” and said it was “fair to say we could not have accomplished what we accomplished without Rahm’s leadership.” He heralded Emanuel as “a great friend of mine,” “a selfless public servant” and “an outstanding chief of staff. I will miss him dearly.”

Emanuel, who choked up several times during the send-off, already seemed to be shifting into campaign mode.

“I’m excited to be heading home to Chicago,” he said, which is “the greatest city in the greatest country in the world.”

Emanuel, a former House Member, praised Obama for staving off “a second Great Depression” and for being “willing to challenge the worn-out ideas and the stale thinking that often stand in the way of progress.”

The Chicago native also thanked his staff for their patience with his well-known foul-mouthed temperament: “I’m sure you’ve learned some words you’ve never heard before and an assortment of a combination of words,” Emanuel said, drawing laughs from the room.

Both Obama and Emanuel also raved about Rouse: Obama called him “a skillful problem-solver” and, because of his vast experience on Capitol Hill, someone “affectionately known as the 101st senator.” Emanuel said he “commands the respect of everyone in this building.”

It remains to be seen who Obama will tap to permanently replace Emanuel. Rouse is known for being shy of the spotlight and may not be interested in a higher-profile slot. Other names that have been floated include Daschle, Deputy National Security Adviser Tom Donilon and Ron Klain, Vice President Joseph Biden’s chief of staff.

Earlier Friday, nearly three dozen top White House aides gave Emanuel a standing ovation — and a dead fish — as he wrapped up his final 8:30 a.m. meeting in the Roosevelt Room. The parting gift, a dead Asian carp, was a joke in reference to Emanuel infamously sending a dead fish to a pollster he didn’t like.

White House aides said Emanuel gave a five-minute, teary-eyed speech to the room as he thanked his staff: “I know I pushed you all really hard. But I did it in service to the president and I believe our country is better off for it,” he told the group.

Council of Economic Advisers chair Austan Goolsbee told Emanuel the fish was from the policy team because “we wanted to give you a going-away present, something to show how we feel about you but also shows we understand your new possibilities. I was the natural go-between: I voted for you all three times you ran for Congress and even in that first primary. So here is your present.”

Liberal groups already have a campaign under way to prevent Emanuel from becoming Chicago’s mayor. The Progressive Change Campaign Committee has been circulating an online petition asking supporters to “Hold Rahm Accountable.”

“I will not support Rahm Emanuel in any future election for Congress, Mayor of Chicago, Governor, or other office. He sold us out on the public option and is a weak Democrat who caves instead of fighting conservatives and corporate power. We won’t forget the choices you’ve made, Rahm,” the petition reads.

As of Friday afternoon, the group had collected 2,996 signatures, according to the PCCC website.

Liberal groups have been critical of the White House for failing to deliver on some of their top priorities, such as a public option in health care reform and breaking up big banks in financial reform.