Named to the job was Carol Elder Bruce, a Washington defense lawyer specializing in white collar crime. She has represented politicians in congressional investigations, served as an independent counsel and was a federal prosecutor in the nation's capital.
Bruce will conduct a preliminary inquiry, which will determine whether a higher level review is warranted or whether the case should be dismissed. The ethics committee determines whether a senator violated standards of conduct.
The Justice Department previously dropped a criminal investigation of Ensign. The department had been looking into whether the senator conspired with staff aide Doug Hampton, the husband of the woman with whom Ensign was involved, to violate federal lobbying restrictions.
Robert Walker, a former chief counsel to the ethics committee and now Ensign's lawyer, said, "The Senate Ethics Committee has assured Senator Ensign that their inquiry remains in the preliminary stage and that the appointment of a special counsel does not change the course of its inquiry.
"Senator Ensign is confident that he complied with all ethics rules and laws, and he is hopeful that this appointment will lead to a more speedy resolution of this matter. As Senator Ensign's office has been doing, they will continue to cooperate with the committee's inquiry."
When Hampton found out about the senator's affair with Cynthia Hampton, Ensign helped line up jobs for Hampton with campaign donors. Federal criminal law bars former Senate aides from lobbying in the Senate for a year after they leave their congressional jobs.
Ensign's parents provided the Hamptons with $96,000 that they described as a gift. The Federal Election Commission has dismissed a complaint against Ensign over the payment.
Ensign admitted in June 2009 that he had the extramarital affair, and that he helped Cynthia Hampton's husband obtain lobbying work with a Nevada company. Ensign said he had undertaken similar efforts for other members of his staff as they moved back into the private sector and that he had violated no law.
The senator has said the affair began during a rocky time in his marriage in December 2007 and continued until August 2008. Cynthia and Doug Hampton left their jobs in May 2008.
Ensign lost a leadership post with Senate Republicans after he disclosed the affair, but he quickly made it clear he had no intentions of resigning his seat.
He also has said he will seek a third term in office. He's up for re-election in 2012.