THERE's a new mood in America. Or, more accurately, a long discredited delusion arisen from the graveyard of bad ideas. In the wake of our triumph in the cold war, Americans are being told we can no longer afford a leading role abroad but must concentrate on problems at home.Portraying themselves as champions of the American taxpayer, some proponents of this new isolationism have called for sharp reductions in American foreign assistance. Their mesmerizing but misguided message has picked up support at a time of recession. The House of Representatives, for example, refused to pass a foreign aid authorization bill similar to one voted for earlier this year by over 100 votes. Foreign aid critics use a handful of faulty arguments. They say foreign aid is a giant giveaway. It's time, say nay-sayers, to end the gravy train. This is nonsense. There may be some unproductive spending. But the bulk of United States assistance to friends around the world is distributed efficiently and honestly. Far from being a giveaway, foreign aid is a tangible expression of American idealism, and it contributes to vital US interests. It goes for humanitarian needs and US security purposes. It plays a role in serving American commercial interests. It helps fledgling democracies. It goes to causes as diverse as narcotics control, peacekeeping, and environmental protection. A frequently lodged complaint is that we can't afford foreign aid. The US, the critics say, has a national debt of $3.6 trillion. The deficit for this year alone is $321 billion. We can't spend money we don't have. Unfortunately for the isolationists, the notion that the wealthiest nation in the world cannot afford a foreign aid program costing $60 per American does not stand up. Foreign aid ($15.1 billion in 1991) is 1.2 percent of the federal budget. Even if we eliminated aid altogether (and no one seriously calls for so drastic a step), the budget deficit would still total $300 billion. We now spend about one-quarter of 1 percent of our GNP on foreign aid. Sixteen of the world's 18 leading industrial countries do better. Americans spend more every year on cosmetics than on foreign aid. We spend twice as much on beer, and three times as much on tobacco. The idea that we can't afford foreign aid is only slightly more respectable than saying we'd prefer to spend our money in other areas. In addition, 70 percent of the money appropriated for foreign aid is actually spent on goods and services in the US, which are then shipped overseas, often on American ships. Our World Bank contributions, moreover, are more than matched by the procurement contracts US companies obtain from that organization. In actuality, cutting foreign aid means taking jobs from US workers and markets from US farmers. In a sense, Americans can't afford to abandon foreign aid. Confronted with such realities, foreign aid critics finally resort to the intuitive contention that America has desperate domestic needs to be funded first. Rather than spending money overseas, they say, we ought to take care of our elderly, homeless, and unemployed. The resolution of our domestic problems must be our first priority. Charity begins at home. But it is not true that we are spending a disproportionate share of national income on foreign aid. Rather, we allot the overwhelming bulk of our federal expenditures - 98.8 cents of every dollar - to areas other than foreign assistance. Slashing the aid budget to increase domestic funds will add only marginally to domestic spending. If we reduced foreign aid by 50 percent and funneled all the savings into health care, it would still generate only a 1.2 percent increase in what Americans now spend on such care. Diverting half the foreign aid budget into education would add less than 2 percent to school spending. But cuts in foreign aid of this magnitude, while doing little to advance our domestic agenda, would drastically undermine our c apacity to promote American interests. Foreign aid works. It saved Europe from communism after World War II. It put Japan, South Korea, and Taiwan back on their feet. It has contributed much to peace in the Middle East. It has shored up fragile democracies in Asia, Africa, and Latin America. It is easing the transition from communism to political pluralism in Eastern Europe. And it has helped the hungry, the sick, the ignorant all over the world. A Fortress-America mentality is no more viable a policy today than it was earlier in the century. A worldwide depression and two global wars should have demonstrated the bankruptcy of a policy of benign neglect as the basis for our relationship with others. The way to avoid a repetition of these tragedies is not to disengage from the world, but to use the opportunity for effective leadership that our cold-war triumph has given. By drastically cutting foreign aid, we would be snatching defeat from the jaws of victory.