The nation's main gauge of future economic activity rose a moderate 0.7 percent in August for the second consecutive month, providing a strong indication that the economy will pick up speed at least through the end of the year.

The August rise in the Commerce Department's index of leading economic indicators -- which is used to gauge economic activity three to nine months ahead -- was the fourth consecutive monthly increase, suggesting to economists that increased consumer spending, construction and factory production will provide a moderate boost to a lethargic economy.

The economy has benefitted from stronger automobile production and sales, attributable in part to cut-rate automobile financing promoted by major auto makers, and from a reduction in imports for two consecutive months, which could provide momentum for boosting growth in other areas, economists said.

However, the auto incentive program is probably only making consumers buy cars now that they would have bought later, thereby reducing growth in coming months, economists asserted.

White House spokesman Larry Speakes said the leading indicators continue "to show the world what can be accomplished when the government lowers taxes and cuts regulations."

Commerce Secretary Malcolm Baldrige said that recent increases in the indicators are consistent with a 4 percent real economic growth rate, a pace forecast recently by the Reagan administration for the last half of the year. Earlier, the administration had said it expected the economy to rebound to a 5 percent rate for the last six months.

"The fourth consecutive rise in the leading index is a welcome sign of better economic performance," Baldrige said. "If the rate of increase in the leading index during the last four months is sustained, the economy will pick up further."

Private economists said that, while the figures indicated solid growth ahead, this growth would not be as strong as the Reagan administration is forecasting, casting doubts on the possibility of reducing the federal budget deficit through economic growth or sharp reductions in the unemployment rate.

"It's good news, certainly, for the rest of the year," David Jones, economist for Aubrey G. Lanston Co. in New York, said of the leading-indicators report. "All in all, I'd say there's a quickening of the pace in the fourth quarter. We're probably in good shape through the end of the year."

A major reason for the upturn is that consumers have resumed some buying, and appear to be increasing purchases of domestic products rather than imports, Jones said. Manufacturers' new orders for consumer goods and materials -- one of the more important of the leading indicators -- rose for the second consecutive month.

"I'd say there's at least a hint . . . that consumers have begun to spend more on domestic goods . . . than imported goods," contrasting with the first part of the year when the large influx of imports "led to the emotional wave of protectionism in Congress," Jones said.

Jones noted the strong pickup in purchases of automobiles after auto makers provided deep discounts on financing. "When you give consumers an incentive to buy, they will buy," Jones said. "That shows that the general picture of the consumer, at least for the near future, is positive."

Many economists were cautious about the report, however. Although the increase in the leading indicators "confirms that the economy will grow somewhat more rapidly in the final quarter, there is little evidence that this will translate into vigorous expansion," said Gordon Richards, director of economic analysis for the National Association of Manufacturers.

Richards said that economic growth will be held back in large part by the high level of consumer debt and by high interest rates. The NAM is forecasting no more than 4 percent growth for the fourth quarter, "and even this moderate pace is likely to be unsustainable in the long run," Richards said.

The increase in the indicators was the largest since last February when the index rose 0.8 percent. The Commerce Department also reported yesterday that it had revised its figure for the increase in leading indicators in June down from 0.4 percent to 0.2 percent. But July's figure was revised up from 0.4 percent to 0.7 percent.

Six of the indicators increased in August. They were: money supply, building permits, average workweek, net business formation, manufacturers' new orders for consumer goods and materials, and average weekly initial claims for state unemployment insurance.

The workweek, building permits, manufacturers' new orders and unemployment claims are considered the most important of the indicators in gauging future factory production, construction and employment.

Those indicators making negative contributions were: stock prices, changes in sensitive materials prices, change in credit outstanding and vendor performance, which gauges the strength of activity by measuring the speed at which companies receive deliveries of goods.

Contracts and orders for plant and equipment, considered another key indicator, was unchanged in August.