President Reagan defended his economic policies in a nationwide radio address yesterday, but a government economist said the Commerce Department will be announcing more bad economic news this week.

While Reagan spoke of "real hope for everyone" because of declining inflation and rising purchasing power, Commerce Department officials said the government's index of leading economic indicators, which is supposed to foretell future economic trends, turned down again in August after pointing upward for four months in a row.

The August figures are to be released this week. Before turning up in April the index had pointed steadily downward for about a year.

Theodore S. Torda, a senior Commerce Department economist, also said yesterday an index of so-called coincident indicators used to measure the current state of the economy had probably fallen in August to a "new cyclical low . . . the lowest point in the recession."

Administration officials have been suggesting recently that the recession has bottomed out and the economy is turning around. But as of August, according to the coincident index, there was "no sign of a turnaround . . . in the economy," Torda said in response to a reporter's question.

Polls indicate that the economy is likely to be the major issue in the Nov. 2 election, and both sides are looking to the government's final economic reports between now and then for comfort. Many experts expect that unemployment figures for September, due Oct. 8, will exceed the 9.8 percent figure for August.

In his weekly radio address, Reagan defended his administration's record in dealing with what he called the "desperate" economic situation inherited from the Carter administration and accused opponents of using the unemployment issue for political purposes.

"I don't believe that it follows that those who took us to the edge of economic Armageddon are automatically best qualified to lecture us now on the most fair, effective way to end the crisis. And when those same individuals charge our administration fights inflation by putting people out of work, I say they're exploiting helpless people for their own political gain," he said.

Reagan cited the administration's record in cutting the inflation rate "more than in half," to 5.1 percent so far this year, and lowering interest rates to 13.5 percent, contending that "more jobs" can only be created by winning the war against inflation.

Reagan never mentioned Democrats by name. But Rep. Peter A. Peyser (D-N.Y.), in the Democrats' response, said:

"It's unbelievable that the president can brag about the worst economic catastrophe since the Great Depression," he said, adding that lower inflation does not necessarily mean a healthy economy. "Today we suffer from the highest unemployment rate and the worst rate of business bankruptcy in 40 years."

Commerce Secretary Malcolm Baldrige reportedly said last week that the forthcoming composite index figures would show a decline.

Torda said reducing inflation has also taken the "steam out of the economy." He explained that the composite figures reflect previously released figures but that the final estimate of the monthly decline in the leading indicators, including 10 categories of economic activity, is not yet complete. One informed analyst estimated that it would be down about one percentage point.

In July, the leading indicators increased by 1.3 percent, continuing an upward trend that began in April. The expected August downturn reflects declines in the average work week in manufacturing, a "very large increase" in initial claims for unemployment insurance, a large decrease in housing permits and another large decline in new orders for consumer goods and materials, Torda said.

He emphasized that a "one-month decline doesn't make a trend . . . . If it continues to drop in the months ahead, there would be serious cause for concern here that the recession is going to drag on." He predicted a possible upturn in September figures because of the recent boom in the stock market.

Torda said the index of "coincident" or current indicators dropped in August largely because of declines in industrial production, non-farm payroll employment, real personal income minus government payments and deflated business sales.