President Reagan launched his fall campaign today before cheering crowds by celebrating the future while pointing with pride to his record of the past.

Reagan told tens of thousands of enthusiastic supporters at an Orange County rally that "America's message to the world," if he is reelected, would be, "You ain't seen nothing yet."

"We've got everything before us," Reagan declared. "We're going to build an economy that you can give to your children and say that it will ensure and fulfill the lives of our next generations.

"We're going to go to work to break the cycles of dependency on government so that free men and woman have the surging spirit of boundless opportunity. We're going to build a peace that won't fail, if we don't fail."

Reagan did not say how he would accomplish any of these goals but his speeches today were permeated with the optimism that dominated his successful campaign against President Carter four years ago.

Buoyed by a national Los Angeles Times poll that gave him a 27-point lead over rival Walter F. Mondale, the president jabbed at the "pack of pessimists roaming the land" and was clearly in high spirits as he campaigned on his political home ground.

When the Orange County crowd interrupted him with chants of "four more years," Reagan smiled and responded, "Okay, you talked me into it."

"We don't seek a victory for any partisan purpose," Reagan told the youthful, crowd at Mile Square Regional Park in staunchly Republican Orange County. "Today, we set out to achieve a victory for the future over the past, for opportunity over retreat, for hope over despair and to move up to all that is possible and not down to that which we fear."

Reagan flew from Orange County for a second campaign speech in Cupertino, 500 miles to the north in the heart of California's prosperous Silicon Valley.

He joked that a computer had been developed in the Silicon Valley that was so advanced it "had actually tallied up all of the promises made by the other side. But when that computer tried to add up all of the tax increases to pay for them, it blew a fuse."

This story was characteristic of the way Reagan treated Mondale, whom he never mentioned by name. Presidential strategists said the approach was deliberate.

According to one campaign aide, the strategy reflected both Reagan's preference for optimistic themes and concern that the blunt, anti-Mondale rhetoric of the Republican convention in Dallas had been too negative.

The new approach emphasizes Reagan administration accomplishments and soft-pedals what the president has in the past referred to as "the failures" of the Carter-Mondale years.

"The future we are building is not about promises, but about promise," Reagan said at a rally here at De Anza Community College. "The future we are building is not for one special interest group, but for all the people."

In both of his speeches Reagan recited a litany of economic recovery statistics, stressing the increase in jobs and the decrease in inflation that has occurred during his presidency. Reagan said that his candidacy presented "a real chance finally to maintain sustained economic growth without inflation."

He told the crowd at the De Anza athletic field, speaking from a stage where a sign proclaimed, "America is Working Again," that "you seem like people who believe in your future, you seem like you're better off than you were four years ago."

In high-technology conscious California, Reagan also stressed his belief in "high tech, not high taxes" and took credit for ordering development of a permanent manned space station, which Mondale opposed.

Reagan also tried to reach out to blue-collar workers, a constituency that in 1980 gave him almost as many votes as it did Carter.

He attacked as "unknowing" a charge made by Thomas Donahue, secretary-treasurer of the AFL-CIO, that administration policies had benefited the rich and failed to create new jobs.

"He might like to know that there are more people employed today than ever in our nation's history," Reagan said.

In an appeal for minority votes, Reagan described the Republicans as the "party of opportunity" but acknowledged that the GOP's appeal was incomplete.

"We will not be satisifed until all Americans understand that they are welcome with us, and belong to us," Reagan said in his Cupertino speech. "The Republican Party won't be complete again until more black Republicans feel it is their home again. The Republican Party won't be complete until Hispanic Americans and every individual in this country understand that they we are the party of opportunity, the part of growth, the party of the future -- and that party is America's party."

Reagan's speeches today were replete with the lines and themes of past campaigns.

But his endorsement of "faith in God" was deliberately general in nature, avoiding the close relationship between religion and politics that Reagan favored in Dallas and that has brought him under fire from Mondale and others.

Reagan's speeches today stressed the domestic economy, as his political advisers have long wanted him to do. But the president also pledged to seek a reduction of nuclear arms if he is elected.

"For the sake of our children and the safety of our earth, we will continue to invite all nations, including the Soviet Union, to join us in keeping the peace and in reducing, and, yes, ridding the earth of the awful threat of destructive nuclear weapons," Reagan said.

Republican professionals in the state said that the Los Angeles Times poll gave Reagan a wider lead than even they believed possible.