WASHINGTON - U.S. President Barack Obama on Friday said a bittersweet goodbye to the energetic and fierce manager of his White House, chief of staff Rahm Emanuel, and elevated a quiet and seasoned adviser, Pete Rouse, to the most important gate-keeping job in American politics.

The departure of one of Obama's key advisers is an unquestioned loss for the president. Obama has relied on Emanuel's intensity, discipline and congressional relationships to keep the White House focused and aggressive.

Other top aides to Obama have announced their departures in recent weeks and such personnel moves are not unusual two years into a presidential term. But they come as Obama's Democratic Party is facing serious challenges in November congressional elections. Widespread losses by the party in the House and Senate could force Obama to make more changes in his staff.

"We could not have accomplished what we've accomplished without Rahm's leadership," Obama said. Emanuel is departing after nearly two grueling years to run for Chicago mayor.

The announcement was such a poorly kept secret that Obama joked it was "the least suspenseful announcement of all time," but it represented an important moment of transition for the presidency.

The mood at the White House reflected that this was no ordinary staff change. Cabinet members and senior staff members packed the ornate East Room, a setting often reserved for visits of heads of state, for the official word that Emanuel, the hard-charging leader of the staff, was on his way out.

Rouse, named interim chief of staff, is a calm, trusted senior adviser to Obama who has spent much of his career as a chief of staff in the Senate. Unlike Emanuel, he rarely talks to the press.

"There is a saying around the White House: 'Let's let Pete fix it,"' Obama said.