President Carter, attempting to focus his campaign on what he called "the real decision facing our country," asserted yesterday that his economic policies can lay the foundation for "an American economic renaissance" leading the country to "economic greatness."

In a 20-minute nationwide radio broadcast paid for by his reelection campaign committee, Carter said that his energy policies have begun to work and that these, coupled with his proposals to revitalize American industry, can mean "a brighter economic future" for the country.

"This is not just a dream," he said. "It is a practical vision that we can bring to life by taking the right actions today -- by investing in our future." b

The speech was the first of three such radio broadcasts the president is to make on Sunday afternoons leading up to Election Day, Nov. 4. The use of the broadcasts as a campaign technique was an outgrowth of the assessment by Carter's political advisers that the president's often strident attacks on Republican Ronald Reagan were backfiring and that he should devote more time to lengthy discussions of the issues with a more "presidential" flavor.

In keeping with new campaign style, Carter yesterday mentioned Reagan only once, and then only to say that the choice in the election "is not just between me and Governor Reagan" but between "two vastly different sets of beliefs." Nor did the president criticize specific economic positions espoused by the Republican nominee. But a major theme of the Carter campaign -- that Reagan lacks the intellectual capacity to deal with the nation's problems -- was an indirect aspect of the president's message.

"The economic challenges which we and the world must face are difficult and complex -- extraordinarily complex," he said.

Asserting the country can overcome its problems "if we recognize them for what they are," he added: "When nations fail to address their challenges realistically, and look for simplistic solutions to their problems, then they run into trouble . . .Are we mature enough and strong enough to accept the realities of the 1980s and to take the difficult but rewarding steps that are needed? Or will we close our eyes and dream of earlier times, simpler problems, and painless solutions?"

Carter is to give what he called a "more detailed" speech on economic policy here tomorrow. In the radio address, he made no new proposals, but said that his energy policies had reduced foreign oil imports by 20 percent in the last year and said it was time to "drive forward" his latest series of economic proposals to end the recession.

The president said "we see the beginnings of recovery" from the recession and he painted an extraordinarily bright picture of the future based on his policies. "It will be a time of enhanced productivity," he said. "I see us turning America farm products into American fuel for American cars and trucks. . . I see an America of full employment . . . It will be a time of purer air, cleaner water and freedom from the threat of toxic wastes."

White House press secretary Jody Powell said the broadcast on the Mutual Radio Network cost the Carter reelection committee $22,000. He said there was no way to determine the size of the audience for the early Sunday afternoon because stations in the network were not required to carry the broadcast.