The sources say Dunn, who is managing director at SDKnickerbocker, had been advising Biden as he made his decision to run for president.

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"She will be a tremendous asset to the team," one source familiar with the decision said.

Dunn served as acting communications director and adviser to Obama in the White House during his first year in office.

Obama on Thursday released a statement praising Biden following his announcement, though he stopped short of endorsing him. Biden said later that he had asked Obama not to endorse him.