President Reagan returned today to his political homeland to launch the fall campaign in a region that has consistently nourished him with votes, financial support and ideological sustenance.

Reagan, who flew here from Washington, will kick off the campaign on Labor Day with a speech to a mammoth rally in Mile High Regional Park, an oasis in a land of subdivisions, freeways and shopping centers.

The setting is in marked and deliberate contrast to the one Reagan used to begin his 1980 campaign, when he was trying to establish himself with Democratic working-class voters.

Then he opened in Liberty Park, N.J., with the Statue of Liberty in the background, before an ethnic audience. On Monday, in the nostalgic beginning of his last campaign, he returns to his political roots in largely white, nativist and Republican Orange County.

By any measure, southern California in general and Orange County in particular is "Reagan Country." Twenty years ago southern California was Reagan's political starting point, and today it remains the 73-year-old president's securest base.

Reagan acknowledged as much when Air Force One arrived here late this afternoon at El Toro Marine Corps Air Station. He told a welcoming crowd of several hundred people, "It's good to be in Orange County, where the good Republicans go before they die . . . good Democrats, too."

Reagan tried out his famous 1964 speech for GOP presidential candidate Barry Goldwater on a southern California network before it was shown nationally.

Two years later, in his first campaign for governor, Reagan stirred the fires of middle-class discontent with the student demonstrations at the University of California in speeches here. He also found a ready audience for his anti-tax and anti-crime messages and his denunciation of "big-brother, paternalistic government."

In 1968, in speeches in Anaheim and Los Angeles, then-Gov. Reagan said that the people should turn to Washington only as a "last resort," the condition he now imposes on a tax increase if he is elected to a second presidential term.

"Those of us who do not see a panacea in the approach of big brother and big government are often accused of oversimplifying, of not being aware of the complexities of modern-day life," Reagan said in Anaheim. "I reject that thesis. I say there are simple answers to many of our problems -- simple but hard."

In 1970, running for reelection as governor, Reagan launched his crusade against the "welfare mess" in southern California, raising the issue that produced the liveliest controversy and the most important legislation of his second term.

Reagan has been rewarded by winning each of the eight times his name has appeared on a ballot in this state -- with margins built in southern California and in the suburbs south of San Francisco, where he is scheduled to give a second speech Monday in San Jose.

In Orange County, the heartland of Reagan Country, Reagan won 72 percent of the vote in 1966, 67 percent running for reelection in 1970, nearly 70 percent against President Gerald R. Ford in the 1976 Republican primary and 68 percent against President Jimmy Carter four years ago.

Reagan polls show him running roughly even or slightly ahead of his 1980 performance in Reagan Country, which stretches in a southerly arc from the smoggy Los Angeles suburbs through prosperous northern San Diego County. The area contains more than half the 4.5 million Californians who voted for Reagan in 1980 and gave him a 17-point margin over Carter.

Orange County is the home of Disneyland, the Los Angeles Rams, the California Angels, the John Birch Society and the Larry McDonald Crusade to Stop Financing Communism.

It is dependably Republican -- nine of its 10 state legislators and four of its five members of Congress belong to the GOP -- and consistently supportive of Reagan.

The county is so noted for its conservatism that a local state senator once joked that he had joined the Birch Society "to get the middle-of-the-road vote in Orange County." But despite its right-wing reputation, Orange County has more often than not been an accurate mirror of white subdivision America, at least in the Sun Belt.

It is a sign of Reagan's popularity here, and perhaps an ominous one for Democratic nominee Walter F. Mondale, that Reagan has been able to maintain his margins of victory in Orange County as the percentage of working-class and Hispanic voters has increased.

Culturally, Orange County remains quintessentially southern Californian. Eight out of nine members of its work force drive to work alone over crowded freeways and are isolated from the poorer communities of south-central Los Angeles.

Most of these freeway dwellers get their news largely from radio and television, the media that Reagan uses most effectively.

Reagan's speech Monday will be a nostalgic event for the candidate, who will be conducting his last campaign no matter what the outcome. Though he now enjoys the favorite's role, political professionals greeted the ex-actor and former Democrat with skepticism when Reagan began to stir the passions of middle-class Californians in the summer of 1966.

In 1980, at Liberty Park, Reagan spoke glowingly of the "brave workers of Poland," introduced the father of Polish trade union activist Lech Walesa and denounced the "sorry record" of the Carter administration.

On Monday, in a festive setting that will feature bands and cheerleaders, Reagan is expected to celebrate economic recovery and restate the contention of his acceptance speech that he is the candidate of the future as well as of the past.

Four years ago, Reagan's Sun Belt strategy was restrained by Carter's relatively high popularity in the South. From the first, Reagan targeted ethnic, blue-collar voters in the industrial states of the Northeast and Midwest.

These voters and these states remain the key battlegrounds, and Reagan plans to spend at least two weeks of September campaigning in the crucial industrial states.

But his strategists this time start with the perception that he has what amounts to a lock on 28 southern, western and plains states -- including California, Texas and Florida -- with 271 electoral votes, one more than required for election.

As a result, Reagan in his Labor Day opening is, as a strategist puts it, "touching base with the people who care about him most." It is not a coincidence that they are also the people who matter most to campaign adviser Stuart K. Spencer, principal campaign speechwriter Kenneth Khachigian and pollster/strategist Richard B. Wirthlin, all of whom call Orange County home.

After his two California speeches, Reagan is scheduled to travel to Salt Lake City, where he is to address the American Legion convention on Tuesday. He is to finish the first week of on-the-road campaigning Wednesday with an economics speech in Chicago.

Reagan is focusing on Illinois, where a combination of black registration in Chicago and downstate concern about his farm policies have put the state's 24 electoral votes up for grabs.

The president will meet with local politicians at every stop and with the Mormon hierarchy in Salt Lake City, but the pace of the week's campaigning will be typically leisurely for Reagan -- and typically focused.

He is scheduled to give only four speeches in as many days away from Washington. All will have a theme designed to dominate the evening news.