IT WAS POSSIBLE to guess at the beginning of the Senate's investigation of the Billy Carter affair that the administration's preemptive disclosures had done most of the committee's work for it. And now that the investigation is all but at an end, this turns out to have been the case. The president, if he responds directly to the committee in some form, may yet add something important. The Justice Department is still looking into whether Billy Carter had something to do with influencing the State Department at one point to approve sale of two jet airplanes to Libya (eventually they were not sold), and whether Zbigniew Brzezinski, in cautioning Billy Carter not to make a prospective business deal with Libya (the deal fell through), revealed classified information. Meanwhile, however, a tentative bottom line can be drawn.

First let's give the administration its due. Aside from the two questions mentioned above, we see no evidence that anyone in the administration did anything overtly criminal or even grossly improper in the classic Watergate abuse-of-power sense. That Billy Carter acted, in several passages, as a shameless hustler is undeniable. But the by now rather detailed record does not show that any official person deliberately -- as distinct from inadvertently, negligently -- served his commercial ends. At the White House, to be sure, a succession of staff aides walked their way with varying degrees of balance between an awareness of the president's personal relationship with his brother and a recognition that public goals also had to be served. At the Justice Department, there was a much sterner sense of duty to institutional and national interest. One can have reservations about some of the choices the attorney general made without thinking he was any the less conscientious in making them.

Nor does the record show that Billy Carter actually influenced American policy, either in the particular matter of the C130s or the general matter of the American approach to the Palestinian question -- although, to judge by the money they shelled out to him, the Libyans may well have thought otherwise.

But that is precisely the rotten element: the Libyans did think otherwise. A fanatical outlaw regime that prides itself on opposition to American values and interests was given to understand that somehow American policy was on the block. It was given to understand this first of all because the president did not see to it, as he well could have, that his brother was absolutely denied the official contacts and accouterments -- briefings, White House limos and the rest -- that were the very stuff of the pretensions to influence that Billy Carter was attempting to peddle. It was further given to understand this by the thoughtless and gratuitous decision to use Billy Carter as a middleman in the Iranian hostage crisis.

Who knows what led Libya's leader, Col. Quaddafi, to boast, just a few days after his man in Washington had been received in the Oval Office, that he had been given presidential assurances of a changed American Mideast policy in 1981? The White House has never stopped denying that such assurances were given, but afterward Billy Carter received from his Libyan friends $220,000 in cash and a pledge of more and a crack at millions in an oil deal.

Mr. Carter has suggested that all of this happened as a result of what he would plainly like others to believe is a virtue of character: his close feeling for members of his family. We think it happened, however, mostly because he has sometimes used members of his family as agents of his office, thereby creating confusion about whether they are acting for him or for themselves. This a failing he has yet even to acknowledge, let alone repudiate.

It can be argued that nothing much changed as a result of the Billy affair except that Billy Carter got richer and Jimmy Carter got sadder: no national interest was harmed. But the manner in which a president conducts his office in inevitably a national interest. That the powers of the presidency have been employed in a certain way in Washington, and interpreted in a certain way in Libya, has an impact extending far beyond the personal or even the political fortunes of one man. The misfortune and the scandal lay in permitting the idea to grow that American policy is up for sale.