Ensign is being investigated for his actions surrounding a sex and lobbying scandal in which he allegedly helped a former top aide, Doug Hampton, get set up as a lobbyist in apparent violation of the one-year lobbying ban. Ensign had had an affair with Hampton's wife, Cynthia Hampton, who also worked for the senator.
The committee's probe is a "preliminary investigation." Sen. Barbara Boxer (D-CA), chair of the committee, opened the preliminary investigation in October 2009.
According to the committee, a preliminary investigation can end in a dismissal of charges or a letter of admonition. If there is evidence of serious violations, however, the committee can open an adjudicatory review. That review could result in a recommendation for a more serious punishment, including expulsion or censure.
Ensign has shaken off other investigations. The Justice Department closed its investigation in December, and the Federal Election Commission -- which was investigating whether Ensign's parents broke campaign finance law when they gave the Hamptons $96,000 -- cleared Ensign in November.
But the Senate Ethics Committee investigation is apparently still alive and well.
Ensign is running for re-election in 2012. He maintains that he did nothing to violate Senate rules.