If the United Nations didn't exist, its die-hard supporters regularly insist, it would have to be invented.

Nonsense. You might want to create something utterly different for the 1980s: something a lot less pretentious, perhaps even a lot of independent institutions tightly focused, each in its own way, on the problems of poverty, hunger, economic development war-and-peace, that are now caught in the United Nations' machinations.

But you would not overexert yourself to reinvent a brawling, sprawling global bureaucracy whose governing bodies routinely violate their own rules -- an institution in which a full two-thirds of its 153 General Assembly votes represents less than 10 percent of the world population and for which a small minority of 30 practicing democracies contributes more than two-thirds of the financial support.

The United Nations, in short, is an institutional outrage, a moral swamp. It operates much of the time by the mob rule of a Third World majority in close alliance with its Communist bloc. By its rules and procedures, it reduces the superpowerful United States to rear-guard blocking actions, by Security Council veto or, as one American representative puts it, by "abstaining things to death."

Nothing would seem to better illustrate its bankruptcy as a peacemaker than the current, nearly obsessive, often-racist campaign to impose a "settlement" in the Middle East at the expense of Israel's security. Already the General Assembly, clearly abusing its power, has "ordered" Israeli withdrawal from the occupied territories of the West Bank, Gaza and Jerusalem by Nov. 15. Without even waiting for that deadline, the anti-Israel cabal launched a drive for economic sanctions. Not only is Israel isolated, but so, for all practical purposes, is its only real protector, the United States.

On this much, both American and Israeli officials agree. But when they are all through reciting the United Nations' weaknesses and liabilities, they bring you back, with a resigned but by no means desperate air, to the fact that the United Nations does, after all, exist. The question, then, is not how to reinvent it, but how to live with it. And on this point, they are astonishingly philosophical.

"This travesty damages the United Nations much more than it does Israel," says the Israeli permanent representative to the United Nations, Yehuda Z. Blum, a feisty academic whose scholarship has centered on the juridical rights and wrongs of the Arab-Israeli conflict. "The radicals and the Soviet Union have taken over this organization," he contends, adding that the West "seems to have given up."

He sees Israel as a "whipping boy" for the U.N. majority, a way of assaulting the West while avoiding a head-on confrontation with the countries that pay the bills.

He worries over the long-term effects of a campaign to "de-legitimatize Israel": "After a while, you become a kind of outlaw and people can act differently against an outlaw." But he counts on American veto-power to protect Israel from the worst.

Around the corner, at the U.S. mission, Blum's American opposite number, Donald McHenry, who moved up from No. 2 to replace Andy Young, sees the U.N. preoccupation with Israel as menacing but manageable, and not necessarily a true measure of the United Nations' net worth to the United States.

"The United Nations, in one sense, is a relection of the world we live in," McHenry argues, "and the fact is that most of the world disagrees with us on the Mideast." He and other officials do not question that the current campaign, by its own extremes, feeds extremism, not least in Israel. But the Americans contend that the Israelis, in their statement policy and their constant reassertion of their claims to Jerusalem, have made their own contribution to the U.N.'s extremes.

The result is what one American official call a "poisoning of the U.N. environment" by the constant raising of the Arab-Israeli conflict in every forum. In Nairobi, for example, the lowest bid on a new building for a U.N. subsidiary agency was rejected because it came from an Israeli firm. The radical Arabs have opposed Alexandria, Egypt, as headquarters for the World Health Organization because of the role played in peacemaking with Israel by President Anwar Sadat. Recent international gatherings of U.N. organizations dealing with international labor and women have been torn apart by irrelevant anti-Israeli resolutions.

Yet McHenry points to the U.N.'s overwhelming condemnation of the Soviet invasion of Afghanistan as evidence that there are pluses. Another he cites is the U.N. relief program in Cambodia.

The United Nations, he thinks, still serves useful purposes: as a safety valve; a medium for negotiation when there's a mutual will; a peacekeeping instrument when the parties both want one; a place to deal, to good effect, with hijacking, a Law of the Sea, economic and energy problems, food, health and other problems where some community of interest exists.

Blum, for his part, reports he's constantly asked in Israel: Why be there? His answer comes in two parts. "Membership means belonging to the international community -- for Jews, having gotten in, it would be foolhardly to give up." And: "When you've always been on the receiving end of condemnation, the United Nations offers at least a right of reply, even if you know you won't win."

The United Nations' 35-year-old dream dies hard.