IT'S STILL POSSIBLE for the federal government to balance its budget by the end of 1980, as President Carter repeatedly promises. But it will take a lot of very good luck, as well as skillful management. It will depend on a lot of things that an American President doesn't control. That's why it's exceedingly unwise for Mr. Carter to persist in turning the balanced budget into a cast-iron commitment - a moral imperative, you might say, instead of merely an economic instrument.

According to the promises, the budget will reach balance in the fiscal year that begins five weeks before the next presidential election. But the current year's deficit will be somewhere around $50 billion, and next year it will go up close to $65 billion. Wait until the year after that, says the President's budget director, Bert Lance. Fiscal 1979 will be the crucial budget and the turning point, he firmly declares.

But the point here is that the federal budget reflects the state of the national economy. In good times, it's sensible and healthy to hold the budget in balance. But when unemployment is high and profits are low, tax revenues fall and the cost of social benefits like unemployment compensation soars. Under those circumstances, a deficit is inevitable.

To reach a balance, the crucial requirement is stable and rapid growth. National output would have to rise at a rate of more than 5 per cent a year for four consecutive years. That kind of growth has happened only once before in this century, in the great boom that began in the Kennedy years and ended in 1966. Last year output rose at a brisk 6 per cent. This year, in contrast, most forecasts - including the administration's - indicate that it will drop to a little under 5 per cent. That's not good enough to bring Mr. Carter out where he wants to be in the fall of 1980.

Growth not only reduces unemployment. Because of the progressive tax structure, prosperity and rising incomes move families into higher tax brackets and that increases tax revenues faster than federal outlays. If you were an economist working out strategy for the President, you would estimate spending for the fiscal year 1981 and then calculate the growth rates necessary to produce that much money in tax revenues.

But the American growth rate is going to be influenced by a long list of pressures that Mr. Carter cannot entirely comamnd. Some of the current inflation comes from abroad - starting with a poor harvest in Russia, perhaps, or oil politics in the Persian Gulf. American economic growth here is affected by the level of exports, which in turn depends on economic growth abroad. But more than anything else, it responds to business investment and companies' assessments of their own futures. Mr. Carter's emphasis on eliminating budget deficits is part of his campaign to reassure the businessmen who control investment.

Federal budget and tax policy is now fairly firmly set through the first half of Mr. Carter's term. Last week Congress established the first round of limits for next year's budget, running up to October 1978. This week Mr. Carter signed the tax-cut bill. The promise of a balance can be achieved only by a rapid increase in revenues over the following two years, even with no new spending programs. The question is not whether a balanced budget is desirable in principle, but whether it's possible to reach by a fixed date in an uncertain world. Mr. Carter has gambled more on this promise than it's worth.