The Senate Ethics Committee, deluged with 60,000 postcards calling for an "emergency investigation" of Sen. Edward M. Kennedy's (D-Mass.) conduct at Chappaquiddick, wants to see an investigation, all right -- of the conservative group that orchestrated the mail campaign.

Committee Chairman Malcolm Wallop (R-Wyo.) and ranking minority member Howell Heflin (D -- Ala.) sent a letter to the U.S. Postal Service last week asking to be kept apprised of any mail-fraud violations on the part of the United States Justice Foundation, a southern California group that launched the anti-Kennedy drive this spring.

A spokesman for the Postal Service said yesterday that a preliminary mail-fraud investigation, based on complaints from people who received the mailing, is under way.

Gary C. Kreep, an Escondido, Calif., lawyer who directs the foundation, said yesterday he was "stunned and baffled" that anyone would be investigating his mailing, and added that he had "absolutely no intention to defraud anyone."

The letter in question is a four-page solicitation for funds and postcards. It has been sent to 600,000 people on various conservative direct-mail lists in the past four months.

The letter recounts in lurid and accusatory detail the events of the night of July 18, 1969, when Mary Jo Kopechne, a campaign worker for Robert F. Kennedy, drowned when a car driven by Edward Kennedy went off a bridge on Chappaquiddick Island in Martha's Vineyard. The senator failed to report the accident for eight hours.

The letter reveals no new facts about the case, but claims that Kennedy has been allowed to cover up his actions, for which he received a suspended sentence on the charge of leaving the scene of an accident.

In the wake of the Senate Ethics Committee's investigation of former senator Harrison A. Williams (D-N.J.) on Abscam bribery charges, the letter calls for a similar inquiry into Kennedy's conduct.

Kreep said the letter has generated roughly $100,000 in contributions, most of which has been spent on additional mailings.

Anne Miskovsky, a spokesman for the ethics panel, said the basis of a mail-fraud charge would be whether the letter is worded in such a way that a recipient might conclude that his money was being used by the foundation to conduct its own probe.

The foundation is actually using the money to lobby the committee, a fact that Kreep claims the letter makes plain.

"This letter was combed over by 12 lawyers before it went out," said Bruce W. Eberle, whose Vienna, Va., direct-mail firm is handling the foundation's account. "There's no problem with it. The whole thing is much ado about nothing."

Eberle, who ranks just behind Richard Viguerie as the nation's largest conservative direct-mail fund-raiser, said the letter went out this spring because "Kennedy is up for reelection and he is always a good target for conservatives." Kreep denied a political motive, saying he was only interested in justice.

As for Kennedy, his press aide, Melody Miller, said yesterday that "we try not to stoop to the level of responding to people who pander to hate."