THE U.S. Senate should probably forget about obtaining any of the notorious Nixon tapes as part of the confirmation hearings of Alexander M. Haig, the secretary of state-designate. The subpoena for the logs of the 1973 Haig meetings with President Nixon should also be abandoned. It is a senseless chase. The subpoena is quite possibly illegal and, if challenged in court, in an almost sure loser for the Senate; the search is itself unfair and has no precedent in any other confirmation hearing.

Legally, the Senate and its Foreign Relations Committee have little to stand on. The Nixon tapes generated their very own Supreme Court decision, the 1974, ruling that directed Nixon to produce some tapes. Nixon lost on the particular taped conversations, but it is not well remembered that he won big on the overall principle. The unanimous court opinion stated emphatically, for the first time in a high court ruling, that the confidentiality of presidential communications is protected by the Constitution. The court said that the presidential confidentiality has "constitutional underpinnings . . . the privilege is fundamental to the operation of government."

The tone and language of the opinion make it clear that the tapes are presumed to be confidential and unobtainable unless there is a presentation of sworn testimony that the discussion might be criminal. John Dean, former Nixon counsel, had testified precisely about specific meetings that were taped.

The Haig-Nixon conversations of 1973 do not come close to meeting such a standard. The senators in fact have said exactly the opposite and have fallen all over themselves praising Haig, saying that there is no evidence to suggest improper conduct on his part. Sen. Charles Percy, the committee chairman, who signed the subpoena for the 1973 logs said: "I have never seen a shred of evidence that would lead me not to believe that he [Haig] will be fit."

The law has for years been clear on at least one point: evidence is never obtainable to prove a charge that is not made. There has to be some reason, clue or hint of relevance or criminal activity before it generally can be subpoenaed. In the tapes opinion, the court applied the standard in the federal rules of criminal procedure about "relevance and admissibility" -- the need to show that the subpoenaed tapes had something to do with the case and could be used in a trial. And the opinion goes on explicitly to note that a "fishing expedition" will not be permitted.

Granted, the Supreme Court opinion deals with a criminal trial only and not with the issue of a subpoena by a Senate committee. But the spirit of the 1973 rulings is clear -- there has to be some established basis for seeking the material, not just a feeling that it might be useful. And another Senate committee, the Senate Watergate Committee, did in 1973-74 try to get some of the Nixon tapes and lost its case.

Whether the Senate likes it or not, Supreme Court might view its pursuit of the tapes as substantially less pressing than that of a prosecutor investigating a crime, especially if there is no specific charge. Carry this thought several steps further. Suppose Haig wrote some letters to his son during the same 1973 Watergate period and talked about the scandal in the White House. Should the Senate subpoena those letters also? No one has suggested that yet, but the Senate would have a better case to get such letters because they are not constitutionally protected as confidential, as the tapes are.

Four years ago when Cyrus Vance and Harold Brown were up for confirmation as secretary of state and secretary of defense, respectively, no one seriously suggested that the Senate subpoena documents from the Defense Department concerning their service during Vietnam War era. But the Vietnam questions were able to be fully explored at their confirmation hearings.

It is little known that, because of the obstructionist action of the White House in 1974, the Watergate prosecutors took a look at the actions of Haig, who was then serving as White House chief of staff, and at the actions of the White House lawyers. Though the Watergate prosecutors were not pleased with the delaying tactics and other stunts pulled by Nixon and his aides, Haig and the lawyers handling Nixon's case were unofficially cleared.

But there is much more than a principle of law involved in this resurfacing of Watergate and the tapes. The larger point has to do with the function of the Senate in reviewing someone's fitness for a Cabinet position. What are the limit? There must be some.

To turn to the practical questions posed by the tapes: the audio quality is terrible. The transcribing would be a nightmare task; the unintelligibles, the ambituities, the vagueness and indirection of Nixon conversations are as maddening as the man himself. If the senators ever got the tapes, they might never be able to agree what actually was on them.

Haig was the chief aide to a criminal president trying to cover up an illegal earsdropping operation. What would it mean if Haig, in an effort to console Nixon or get out of Nixon's office (Nixon was legend for keeping aides for hours rambling on), said something encouraging that would look bad now? prosecutor Leon Jaworski's characterization of Haig's White House service during Watergate as "heroic." Haig was in tough spot and played out many contradictory strategies on Watergate, saying and doing many often contradictory things. He is probably a shameless self-promoter, and carried situation ethics to the point of making it a personal character flaw.

Haig, nonetheless, did keep a rickety and criminal ship of state afloat and helped ease Nixon out of office. To my mind, he should be neither hailed nor strangled for that role, only held accountable. That has happened and is happening. But let it happen without the tapes.