The watchdog group Citizens for Responsibility and Ethics in Washington has served notice that it will file an ethics complaint against Sen. John Ensign (R-Nev.), and we concur — without prejudging the case — that issues arising from his sexual affair with a former aide deserve examination.

Regardless of the outcome of an Ensign probe, however, we are concerned that — as blogger Paul Blumenthal has written on the Web site of another watchdog group, the Sunlight Foundation — the Senate Ethics Committee is “a black hole where investigations go to die.—

The committee never took action in the case of former Sen. Ted Stevens (R-Alaska), despite abundant evidence that an oil contractor rebuilt his house and gave him other gifts never reported on his financial disclosure forms.

The overturning of Stevens’ guilty verdict — owing to prosecutorial misconduct — does not erase the Ethics Committee’s dismal failure to police what seems an obvious violation of Senate rules.

More than a year ago, the committee got a complaint from CREW concerning unusually favorable mortgage terms accorded Sens. Chris Dodd (D-Conn.) and Kent Conrad (D-N.D.) by the now-defunct Countrywide Financial. Not a word has been heard about that probe.

Dodd is also the subject of a complaint, filed in April by yet another watchdog group, Judicial Watch, growing out of his purchase of — and apparent profit on — property in Ireland from the business partner of a person convicted of stock fraud for whom Dodd secured a pardon.

Undoubtedly too little time has passed for a thorough probe of that case — or of the circumstances under which Sen. Roland Burris (D) was appointed by disgraced former Illinois Gov. Rod Blagojevich (D) to fill out the term of President Barack Obama.

But it’s important to keep track of the cases referred to the Ethics Committee and to urge that — someday — there be an accounting of them.

In Ensign’s case, CREW’s complaint is expected to be based, first, on charges by his former administrative assistant, Doug Hampton, that he and his wife, former political aide Cindy Hampton, were fired because of Ensign’s affair with Cindy Hampton.

CREW may also raise the possibility that Ensign made payments to one or both of the Hamptons to buy their silence about the affair, publicity around which has seriously damaged Ensign politically.

Ensign did increase Cindy Hampton’s pay while the affair was under way — he says, because her political role expanded. And he did help secure jobs for Doug Hampton after his dismissal.

Ensign charges that Doug Hampton made “exorbitant demands— for money, and Hampton admits he sought “restitution— from Ensign. The Ethics Committee needs to determine whether any money was paid and from what accounts.

And, when it is done, the committee should report its findings — to punish or clear Senators now under suspicion of improper conduct and to demonstrate that the committee is not a “black hole.—