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"Mick has done an outstanding job while in the Administration...I look forward to working with him in this new capacity as we continue to MAKE AMERICA GREAT AGAIN!" Trump tweeted.
SEE ALSO: Who is Mick Mulvaney, Pres. Trump's new acting White House chief of staff?
Trump deemed Mulvaney his "acting chief of staff" but it was not immediately clear what that meant for the length of his tenure.
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Mulvaney succeeds Gen. John Kelly, who is set to depart at the end of the year. Kelly's departure was announced Dec. 8.
The president's first choice was Nick Ayers, the vice president's chief of staff, who bowed out after being unable to come to an agreement on how long he would serve in the post.
The Associated Press contributed to this report.