Anybody taking up for Jimmy Carter in the matter of brother Billy ought to get hazardous duty pay. Just as the president's defenders seem to be rolling back the enemy, they get clobbered by friendly fire -- by information needlessly withheld or affirmations previously denied.

Because there may be no limit to this administration's capacity to self-destruct, predicting the outcome is also hazardous. That said, as of this writing it seems to me that at least one large element of what the Republicans so lovingly call Billygate -- the part having to do directly with the White House use of Billy Carter's Libyan connection -- may turn out to be something of a bust.

Not as theater, mind you, but on the merits, when the case is spread out before the Bayh committee and elsewhere. As the questioning begins to turn on whether it was the president or his wife or his national security adviser -- or Miss Lillian -- who first thought of using Billy as a go-between with the Libyans, the suspicion may begin to set in that the Watergate analogy, so assiduously cultivated by those who would like to make Richard Nixon look better by making Jimmy Carter look worse, simply won't wash.

When you set aside the issues having to do with Billy's independent transactions with the Libyans and simply focus on the alleged White House abuse of his Libyan connections, what do you have to work with -- scandalwise?

Some of the story is not in dispute. Last November, right after the hostages were taken, Billy Carter was asked to set up a meeting between White House national security adviser Zbigniew Brzezinski and Libya's chief representative in Washington. At that meeting, which Billy attended, Brzezinski sought the help of Libya's strongman, Muammar Qaddafi -- a fanatic much given to terrorism but also on close terms with Ayatollah Khomeini -- in securing the hostages' release.

Subsequently, President Carter met with the same Libyan diplomat, Ali Houderi, allegedly both to pass on his thanks to Qaddafi for his public and private representations to the ayatollah and to protest the Libyan sacking of the American Embassy in December.

Last month, Billy Carter (on his initiative) met with Brzezinski and White House counsel Lloyd Cutler, at which time Billy was put in touch with a Washington lawyer to represent him in sessions with Justice Department investigators.

From this, essentially, allegations have developed to the general effect that the White House deliberately set out to build up Billy in Libyan eyes, the better to enrich him. It is suggested that the president stood to gain as well, if Billy used his Libyan loot to pay off his indebtedness to the family peanut-farming operations.

But this charge, White House officials insist, doesn't really meet the famous Howard Baker what-did-he-know-and-when-did-he-know-it Watergate test. fWhen Billy was used as an intermediary to the Libyans in November, the White House had no reason to know he was on the take from the Libyans because, in fact, he wasn't. The first payment under the $500,000 "loan" did not take place until the turn of the year.

Was Billy Carter the right intermediary to begin with? Maybe not. But those were desperate days when even longer-shot efforts were being made, and Billy was one of Libya's few known American friends. A case could be made that he, Qaddafi and Khomeini at least had in common, if nothing else, a certain loose-cannon craziness.

As for the Brzezinski-Cutler-Billy Carter meeting, the White House version suggests an unsuspected sense of responsibility on Billy Carter's part. As Cutler has it, the president's brother wanted only to know if he was free, in the interests of national security, to tell the Justice investigators about his role in the approach to Qaddafi on the hostages.

Cutler had been brought in when it became clear to Brzezinski that Billy was raising legal questions having to do with the investigation, and the Cutler/Brzezinski advice to Billy was to tell the investigators everything. It was in this context that Cutler raised the question of a lawyer, offered a choice of severaland introduced Billy on the phone to the one he had selected.

Unusual? Perhaps, but not on its face implausible.

This is not to say there isn't more to the Billy Carter affair (the discussion of the investigation by the president and the attorney general, the suspicions that Billy may have been tipped off about the investigation by the White House) or more to the Libyan connection than meets the eye.

It is simply to say that right now the uproar over the Libyan connection looks all out of proportion to the available evidence. To the extent that this continues to be the case, the "important foreign policy considerations" that Sen. Robert Dole (R-Kan.) sees in the Billy Carter affair may have to do not with the Libyan matter he obviously had in mind, but with this country's prestige as a world power capable of dealing seriously with real problems.