For more than four years now United Nations-baiting has been the Reagan administration's favorite international sport. But it's absurd to be confrontational.

On any given day, the United Nations looks less like the last-great-hope-for- peace and more like a squalid bully pulpit from which rinky-dink mini-states can bully-rag America. It is a politically stacked deck. The United States is often at the mercy of a majority of international juvenile delinquents.

It's harder for the president of the United States to be merely confrontational when he makes his ritual pilgrimage to address the General Assembly and a modicum of statesmanship is called for. Rarely has the assignment been as difficult as it will be for Ronald Reagan this week.

Not the least of the reasons is that the United Nations is awash in nostalgia, celebrating its 40th anniversary, summoning the high hopes and rich promise that attended its birth. Presidents and prime ministers are parading on stage plighting their troth to peacekeeping, the rule of law and all the dreams and designs (in large part American) of the U.N. Charter.

But the challenge is compounded for a larger reason: Rarely, if ever, has there been such a gap between ritual U.S. plight-trothing and U.S. practice. The United States is not alone in this deficiency, as you will have noted in the name-calling, acrimony, cross-purposes and apparent double-crosses of supposed friends in the efforts to deal with the crisis of the Achille Lauro.

But that is not the heaviest baggage Ronald Reagan will carry to the General Assembly; respected international lawyers do not fault the United States for dispatching its own posse after the hijackers. The heaviest burden lies in a general perception that the chief architect of the United Nations -- the role model of democracy, the self-professed pacesetter in all things good and right -- has lost its faith in the rule of the law.

Nothing better illustrates the point than something the United States did on the very day the Achille Lauro was hijacked and the Reagan administration began to appeal for strict application of international law. On that day, the United States said, in effect: Stop the World Court, we want to get off.

That may sound like no big deal; the World Court is an obscure body with scant enforcement powers. But if the United Nations stands for anything, the World Court must stand for something as its judicial arm.

Yet two weeks ago, the Reagan administration ended 39 years of U.S. adherence to the court's jurisdiction -- except as it may please the United States in cases of its own choosing. It figured. The United States decided early this year to boycott the court's proceedings on the charges brought by the Sandinista government of Nicaragua that the United States was in violation of international law in its mining of Nicaraguan ports and its support of the Nicaraguan counter- revolutionaries (contras). The latest move does more than revive international criticism of America's unilateral Central American policy. It is a vote of no confidence in international law.

The administration's argument hardly helps. Less than a third of the world's nations accept the court's jurisdiction, the State Department argued, "and the Soviet Union and its allies have never been among them." If you won't be more like us, in other words, we will be more like you, America would seem to be saying to the world.

But no matter. The question the president will have to answer this week runs deeper. It may well be that the United Nations in its present form is a lost cause. Institutionalized rule of law may be a dead letter. But if that's so, a nation laying claim to world leadership cannot convincingly praise its "proud history of promoting conciliation and helping keep the peace," as Reagan did in his General Assembly address a year ago, while opting out of its institutions and engaging in empty you're-another-and-so's-your-mother catcalling in U.N. debate.

Selective and self-serving exploitation and/or denigration of the only world organization endowed with peacekeeping potential only delays the day when leaders of the stature of the original founders will sit down soberly and try again.