Economy loses steam.

There are growing signs that the United States economy is slowing, although many forecasters say it is still chugging along at a stronger-than-average pace. ''The economy is growing more slowly now than it did a year ago, and we believe it will be growing more slowly in a year than it is now,'' says Donald Straszheim, vice-president at Wharton Econometric Forecasting Associates.

The expected slowdown could have a variety of effects on consumers' pocketbooks, but most of them are likely to be felt after the election, forecasters say. On the downside, reductions in unemployment are likely to come more slowly and fewer workers will be offered overtime, they say. On the positive side, the slowdown is expected to reduce upward pressure on interest rates and consumer prices.

The latest sign of the slowdown came Wednesday, when the US government reported that the index of leading indicators fell in July for the second consecutive month. The index is designed to predict the economy's future course. July's 0.8 percent decline followed a 1.3 percent drop in June, the first back-to-back drops since December 1981 and January 1982, when the recession was beginning to fade.

The decline in the index is ''probably favorable,'' because among investors ''it will be interpreted as a sign that the economy is slowing and will avoid heating up inflationary forces,'' says Sandra Shaber, senior economist at Chase Econometrics.

White House spokesman Larry Speakes said the decline signaled ''some moderation ahead in economic activity'' that should help ease interest-rate pressures. He added that the economy ''remains strong and growing for the foreseeable future.''

''The pause in growth being signaled is not occurring today. Instead it will occur after the election,'' notes Roger E. Brinner, director of US forecasting at Data Resources Inc. (DRI). His firm sees inflation-adjusted growth slowing from a 7.6 percent annual rate in the second quarter of 1984 to 3.3 percent in the fourth quarter. In the final three quarters of '85, DRI expects the economy to grow at an annual rate of 1 percent or less each quarter.

Weakness among interest-rate-sensitive sectors of the economy - like trade and housing - is one reason forecasters say they expect the recovery to slow. Signs of that weakness have been apparent, in varying degrees, in recent statistics.

For instance, the government Wednesday said that in July the US posted a record $14.1 billion trade deficit - the gap between what Americans purchased from overseas and what US was able to sell abroad. So far this year, the US trade deficit is $73.8 billion. Experts say they expect it to end the year in the $110 billion-to-$130 billion range. By contrast, last year's then-record trade deficit hit $69.4 billion. The government estimates that each $1 billion in sales lost to overseas companies could have produced 25,000 jobs in the US.

US trade reacts to interest rates indirectly. Higher US rates make dollar investments more attractive to foreigners. This raises the dollar's value relative to other currencies, making it cheaper for US citizens to buy foreign goods and harder for US companies to export.

Some signs of weakness are also appearing in housing, another key interest-sensitive sector, although the growing use of adjustable-rate mortgages has helped offset the impact of high interest rates. Adjustable-rate loans carry lower initial payments than fixed-rate loans. The National Association of Realtors reported Tuesday that resales of existing homes dropped 6.1 percent in July, to 2.78 million units, although sales for the year as a whole are still ahead of 1983's pace.

On Wednesday, the Commerce Department said July sales of new homes were unchanged from June's strong seasonally adjusted annual rate of 633,000 units. The average price of a new home, however, fell by $2,200 to $95,600.

The slower growth economists expect will mean slower gains in the battle against unemployment, which could begin to climb again in the middle of 1985, says DRI's Mr. Brinner.

Less robust economic growth could also ease the normal upward pressure put on prices as an economic recovery matures. But some accelerating price trends have begun to appear in ''bread-and-butter categories'' like food, housing, and apparel, notes Allen Sinai, chief economist at Shearson Lehman/American Express. And recent documents released by the Federal Reserve Board show the central bank's concern that growth will be far enough above the economy's long-term 3.0 to 3.5 percent annual growth potential to spur rising prices in 1985.

One might expect slower growth to take some pressure off interest rates. While some forecasters say they expect rates to ease slightly for the rest of 1984, most of those surveyed by Money Market Services Inc. say they expect slightly higher rates by year's end.

The current consensus among forecasters is that interest rates could rise still further next year. Faster-than-normal growth almost always leads to rising rates, according to Goldman, Sachs economist Robert M. Giordano. Other economists say they expect that businesses will have to borrow more money to pay for new factories and equipment. That borrowing, coupled with government borrowing to finance the deficit, could also put upward pressure on interest rates.