A top aide to a former Republican congressman from Oklahoma has pleaded guilty to a conspiracy to defraud the House as part of the Jack Abramoff lobbying scandal. The aide, John Albaugh, who was chief of staff to Representative Ernest Istook, told a federal judge that he was guilty of accepting gifts in return for official favors. Mr. Albaugh faces up to 24 months in prison but the sentence could be reduced if he cooperates with the government’s investigation.