No Supreme Court Justice named in the past half-century even begins to rival in importance the president who made the appointment. So put aside as momentary over-excitement the spate of recent comment asserting that presidents chiefly make history by their selections for the court.

Even in that perspective, however, President Reagan's choice of Sandra Day O'Connor cast a long shadow. Not only does the president break a pattern of sex discrimination; he also delivers on a campaign promise in a way that fosters faith in the system. Finally, he shores up the court -- or at least works against deterioration -- by naming a judge with affinities to its shifting center as against its two extremes.

The honorific status of the court, whatever else may be in question, does not admit doubt. The Supreme Court is the most dignified of our institutions, the holy of holies in the American system, the "ark of the national covenant." Groups accustomed ot viewing themselves as outsiders inevitably attach high importance to being included in. The nomination of Louis Brandeis was a milestone for American Jews, as was the designation of Thurgood Marshall for blacks. The just claim of women to a more equal role thus finds a fit cause for satisfaction in the nomination of O'Connor to the highest tribunal in the land.

President Reagan, of course, did not exactly promise that he would name a woman to the court. But he did commit himself in the campaign to fill" one of the first Supreme Court vacancies" by "the most qualified woman I can possibly find." Designation of a man at this time, however meritorious, would have looked like the first installment on a breach of trust.

But trust is perhaps the single most important bond between the leader and the led in modern society. The complexity of affairs has made it well nigh impossible for most of us to make confident judgments about the working of government. The best we can hope to achieve is a sense of rapport with an individual leader.

So when a leader goes back on what is perceived to be a pledge, the system as a whole suffers. When it is possible to deliver, as Reagan did deliver in naming O'Connor, we all benefit. Which is one reason why those seeking to fight the choice on the issue of abortion face such an uphill battle.

As to the court itself, it has recently been marked by vacillation, narrow decisions, tie votes and a record number of plurality decisions without any majority view. The dominant pattern of the past two years, largely unarticulated, has consisted of a ceding of authority once claimed by the court to the president, the Congress and the states.

Behind the uncertainty and effacement lies a divided court. Two justices -- William Brennan and Marshall -- are liberals of the old school, partial to the rights of individuals and minorities, and determined to assert the claims of the federal government against such bodies as the states, law-enforcement agencies and the big corporations. Two others -- Chief Justice Warren Burger and Justice William Rehnquist -- are liberals, coming down on the other side of those issues most of the time.

The floating center includes Justices Byron White, Harry Blackmun, Lewis Powell and John Paul Stevens. Potter Stewart, whose resignation opened the door to O'Connor's nomination, was a central member of the center. But the center, though a majority, backed and filled and chopped and changed, and set down no clear guidelines. A typical example is Stewart's famous -- and to my mind wrongly praised -- remark about pornography that "I know it when I see it."

Nobody can assess how new justices will interact with a sitting court. But everything known about O'Connor tilts her toward the center.

She is not identified with any ideological grouping. She has moved on the margin in such matters as abortion and the Equal Rights Amendment. Though a Republican, she was appointed to the Arizona Appeals Court by a Democratic governor, Bruce Babbitt. Earlier she had experience in the building of majorities and the art of compromise as a leader in the Arizona Senate.

Whether O'Connor will be able to galvanize the center of the court, find a rationale for what often seems arbitrary and a tongue for ideas that remain mute is very much in doubt. She lacks experience in the federal system. "Bright" and "crisp" are the words used about her by her friends -- not "deep" or "thoughtful."

But the opportunity is there, and plenty of time for learning and reflection. At the very least, it is hard to see how Judge O'Connor can do harm to an institution that is precious in no small measure because it is revered.