THE UNITED STATES' deficit in its foreign trade has become gigantic and ominous. The trade deficit isn't one of the numbers to which Washington reacts sharply, like the inflation and unemployment rates. But it ought to be. The Commerce Department has just published the trade data for May: Americans imported $30 billion worth of goods and exported $17 billion worth. This country is now riding a great boom of spending and consumption, much of it with borrowed money. A crucial part of that borrowed money comes from abroad. As credit-card statements sometimes say, you don't have to make any payment this month but interest charges will accumulate.

Americans as a society are now consuming more than they produce at a rate of about $120 billion a year. The world's largest creditor ntil several years ago, the United States by now is probably a debtor. The statistics here aren't entirely reliable, but you can see the net earnings on foreign investment dropping like a stone. In the peak year, 1981, this country had a net income of $34 billion on its foreign investments -- meaning that it collected $34 billion more abroad than it paid out on foreign investments here. That income was down to $19 billion last year, and was running at a rate of $10 billion in the first quarter of this year. By the end of the year it is likely to be zero. By this time in 1986, if it continues on its present path, the United States will be borrowing abroad to pay interest on its foreign debt. That's the road to Argentina.

"Overconsumption -- unduly raising current consumption at the expense of future levels -- is like a time bomb," observes one close watcher, John D. Paulus, the chief economist for the investment banking firm Morgan Stanley and Co. "As it ticks away it seems to be hurting nobody, but ultimately it has great destructive potential." In time, he points out, it would lead either to a fall in the dollar with higher inflation, or rising interest rates with sluggish economic growth -- or both.

If foreigners eventually tire of lending to the United States, it will have to meet its debt obligations by running a trade surplus. Reversing the present congenial pattern, it will have to sell more abroad than it imports. That has happened to the Latin American countries, as they struggle with their debts, and it has not been a comfortable change.

For the present, life is prosperous and the sun is shining for most Americans. But at some point in the coming years, and no one knows when, the country's credit will run out. Then Americans will find themselves working harder and earning less. Why? Because they will be paying for the great surges of foreign goods that they are currently buying on the installment plan.