Dunn joined the administration in the spring with the understanding that she would stay only a few months because of family considerations. In her time with the Obama administration, she came to epitomize a more combative White House approach in dealing with critics.

She advocated a "rapid response" to counteract allegations that she deemed to be meritless. She also pushed for President Obama to do more interviews, said an administration official who asked not to be identified when discussing strategy.

Pfeiffer, a veteran of Obama's 2008 campaign, also favors aggressive means of communicating the White House message.

Dunn declined to comment yesterday on her departure. Her work was largely behind the scenes, but she found herself in the headlines when she asserted that Fox was not a traditional news-gathering operation.

Last month, she told CNN that Fox was "part of the Republican Party." She said the network's practice was to "take their talking points and put them on the air. Take their opposition research and put them on the air, and that's fine. But let's not pretend they're a news network the way CNN is."

That touched off a small-screen feud. A Fox host, Glenn Beck, aired footage of Dunn addressing a high school graduation ceremony in June and describing Mao Tse-tung and Mother Teresa as two of her "favorite political philosophers."

Beck said on his show: "The guy responsible for more deaths than any other 20th-century leader is her favorite philosopher!"

Dunn later took a swipe at Beck on CNN: "The use of the phrase favorite political philosophers was intended as irony, but clearly the effort fell flat - at least with a certain Fox commentator whose sense of irony may be missing."