Sen. John Ensign is expected to announce at an afternoon news conference in Las Vegas that he will retire rather than face a brutal 2012 re-election campaign, according to knowledgeable sources.

The Nevada Republican, who has been embattled since revealing he had an affair with the wife of a former aide, was facing a possible primary challenge from Rep. Dean Heller (R). Heller is now expected to run for the open Senate seat.

Ensign was facing growing pressure to exit the Senate race, and he has mounting legal bills related to his admission of the extramarital affair. He is the eighth Senator and third Republican to announce retirement this Congress. To see who else is leaving, check out our Casualty List.

Roll Call Politics currently rates the Nevada Senate race as a Tossup.