As the United States economy slows, the negative impact of the US trade deficit will become more apparent, experts say. Less robust economic growth and the deteriorating US trade position were clearly displayed in statistics the government released Wednesday.
The government's index of leading economic indicators rose a weak 0.4 percent in September, after revised figures showed that the index fell in each of the previous three months. The figures for September are incomplete and subject to revision.
The index is designed to signal the future course of the economy. A rough guideline is that three months of consecutive decline signals a recession. Three times, however, since World War II the index fell three months in a row without being followed by a recession.
''We don't believe it tells us a recession is on the horizon,'' says Donald Straszheim, vice-president at Wharton Econometric Forecasting Associates. The belief that three months of decline signals a recession is '' a rule of thumb and nothing more,'' he says.
The weak showing of the index is a sign that the economy's pace of growth has slowed and will slow more in 1985, Mr. Straszheim says. He expects inflation-adjusted economic growth rate to slow to a 3.6 percent pace in 1985, after surging 6.9 percent this year.
David Wyss, senior vice-president at Data Resources Inc. (DRI), another forecasting company, says the weak economic indicators signal a ''growth recession.'' That scenario includes continuing economic growth that is too feeble to keep unemployment from rising.
DRI predicts economic growth next year of 2.5 percent and expects the unemployment rate at the end of 1985 to be 7.6 percent, vs. the 7.2 percent it expects by the end of this year.
White House spokesman Larry Speakes took an optimistic view of the index. The leading indicators, he said, ''show the economy is maintaining its strength and heading in the right direction.''
Still, private forecasters say one reason the economy has slowed is that a record amount of US buying power is being used to purchase foreign-made goods.
The US merchandise trade deficit, the difference between the value of what the US sells abroad and what the nation imports, hit $12.6 billion in September, the second-largest deficit ever.
September's red ink brought the total trade deficit for the year to $96.3 billion. That figure is higher than the total of any previous year. The Reagan administration is predicting a trade deficit of $130 billion for this year.
In September American exports rose just 0.8 percent from August, to $18.2 billion. Imports rose 10.5 percent from August, to $30.8 billion.
One bright spot in the import picture has been falling oil prices, which have helped curb the cost of oil imports. Oil imports cost 5.4 percent less in September, as the average price for a barrel of oil dipped 41 cents, to $28.70.
Experts say it is too early to tell how oil prices wll be affected by the Organization of Petroleum Exporting Countries' (OPEC) announcement Wednesday that it would cut oil production by 1.5 million barrels a day, or 9 percent. The move is a bid by the organization to reverse recent oil price cuts.
Saudi Arabia agreed to absorb almost half of the oil production cuts. Other OPEC nations, except Nigeria and Iran, have agreed to smaller production cuts.
The impact ''depends on whether they can make the cuts stick,'' DRI economist Wyss says. ''They haven't had a good record'' on holding to production cuts, he notes. He figures the best OPEC can do is to stabilize prices at current depressed levels.
Many forecasters expect the US trade deficit in 1985 to be even larger than this year's. The reason is that 1985 is expected to start with the dollar well above its early '84 level.
The strong dollar is a key reason for the poor trade picture, since it makes US goods more expensive overseas while making imported goods cheaper for Americans to buy. The prospect of a higher trade deficit in 1985 has recently prompted forecasters to lower their forecasts for economic growth in 1985.
''As a result of the poorer prospects for the foreign sector, we reduced our estimate of real GNP (gross national product, the value of the output of the nation's goods and services) between the fourth quarters of 1984 and 1985 to 3.2 percent, from 4.2 percent,'' Gary Wenglowski, director of economic research at Goldman, Sachs & Co., wrote clients recently.
''Worsening trade is an important element in slowing the economy,'' Mr. Straszheim says.
DRI estimates that GNP would have risen 9 percent this year, but weak trade performance pulled that down to about 7 percent. Next year further deterioration in trade will trim about three quarters of a percentage point off GNP growth, which otherwise would have been 3.25 percent.
''Trade next year is the difference between unemployment going down next year and going up,'' as many economists expect, Mr. Wyss says.