An index designed to forecast the economy's near-term performance rose in July for the sixth straight month, but two indicators of actual economic performance in July declined, the Commerce Department reported yesterday.

The department's index of leading economic indicators rose 0.5 percent in July and a revised 1.0 percent in June, more than the 0.8 percent previously estimated for that month. The last time the index fell was in January.

New orders for manufactured goods, meanwhile, fell 0.2 percent in July as an increase in orders for nondurable goods was offset by a decline in orders for durables. And construction spending fell 0.2 percent, the second decline in two months.

The index of leading indicators is a composite of nine measurements that, taken together, have demonstrated a credible record in predicting economic behavior. Economists said the increase in the index was another sign that the economy is likely to continue expanding steadily through 1987.

"When you put the July numbers together with June, the combined effect would be consistent with {annual inflation-adjusted} economic growth of somewhat stronger than 3 percent," said Donald Ratajczak, head of the Georgia State University Forecasting Project.

As the government was reporting the rise in the index, White House spokesman Marlin Fitzwater celebrated what he said was a record-length economic boom, hoisting a bottle of champagne at the daily press briefing in Santa Barbara, Calif., near where President Reagan is vacationing. Fitzwater's jubilation was a bit premature, however: The expansion will not become the longest peacetime growth period this century until the end of September.

The four components of the index that improved tended to be more financial than industrial. They were: vendor performance (a measure of the number of companies receiving slower deliveries from suppliers), stock prices, prices of sensitive materials, and orders for plant and equipment.

The three declining components were manufacturers' orders for consumer goods and materials, building permits and the inflation-adjusted money supply. The other two indicators, weekly claims for unemployment insurance and average hours worked per week, did not change.

Roger Brinner, chief economist of Data Resources/McGraw-Hill Inc., said the slowdown in deliveries and the rise in commodity prices registered by the leading indicators might have raised fears that inflation would intensify if they had appeared a decade ago. But in today's economy, Brinner said, U.S. consumers probably would increase their purchases of imported goods if there were signs of rising domestic prices.

Despite the fall in orders for manufactured goods, economists found signs of improvement in the manufacturing sector. Orders for nondefense capital goods rose 0.8 percent in July, while defense-related orders were down 6.0 percent. Ratacjack noted that the increase in nondefense capital goods orders in the last 12 months has been a strong 5.1 percent.

However, inventories -- the amount of goods on the shelf -- grew 0.5 percent in July, a possible sign that manufacturers have been overproducing. Michael Evans, president of Evans Economics, said this stage of the business cycle is not normally the time when an inventory rise would be expected.

Evans also predicted that the decline in the construction sector -- generally considered the result of overbuilding and last year's changes in the tax code -- would reduce third-quarter economic growth by about 0.5 percent. In inflation-adjusted terms, construction spending fell 0.8 percent last month.

The new figures indicated that spending on nonresidential construction in July was 8 percent below its level of a year ago, while spending on one-unit residences was 10 percent higher than it had been in July 1986.