"In contrast to the 2006 dismissals, Presidents rarely dismiss U.S. attorneys they appoint.[59][60] Kyle Sampson, Chief of Staff at the Department of Justice, noted in a January 9, 2006, e-mail to Harriet Miers: "In recent memory, during the Reagan and Clinton Administrations, Presidents Reagan and Clinton did not seek to remove and replace U.S. Attorneys they had appointed, but instead permitted such U.S. Attorneys to serve indefinitely under the holdover provision." (underlining original).[62] There is no precedent for a President to dismiss several U.S attorneys at one time while in the middle of their terms."
also from the above link