** FILE ** Facing an ethics investigation by a special counsel, Sen. John Ensign, Nevada Republican, seen here in June 2009, announced on Thursday, April 21, 2011, that he would step down from his Senate seat. He earlier said he would not run for re-election. (Associated Press)

** FILE ** Facing an ethics investigation by a special counsel, Sen. John Ensign, Nevada Republican, seen here in June 2009, announced on Thursday, April 21, 2011, that he would step down from his Senate seat. He earlier said he would not run for re-election. (Associated Press)