When Carey McWilliams left California 29 years ago for a week's assignment with The Nation that lasted a generation, many of the rich and powerful of this state were more than glad to see him go.

McWilliams remembers when his books were banned or burned in California. He remembers, too, the time when bricks shattered a classroom window where he was giving a speech during World War II urging that interned Japanese-Americans be allowed to return to their California homes. At one time, after the publication of his historic book, "Factories in the Field," the Associated Farmers of California described McWilliams as "Agricultural Pest No. 1 in California, out-ranking pear blight and the boll weevil."

Now, these incidents are only bittersweet memories for McWilliams as he returns to the city of his first books and early manhood.

Today, in cavernous Union Station where McWilliams once watched the lobbyists riding off to Sacramento and Washington, the old radicals who revere McWilliams and the young ones who never knew him will gather for a celebration honoring him as the genuine American radical that the 73-year-old McWilliams considers himself to be. Regardless of their age, they knew of McWilliams' reputation as the muckraking and recently retired editor of the influential and militant New York magazine, The Nation.

But McWilliams is also known as the man who, more than any other journalist or historian, long ago defined for Americans everywhere the mythic state of California -- that extraordinary place where the moods and movements of today become the national reality of tomorrow.

"In California the lights went on all at once, in a blaze, and they have never been dimmed," McWilliams wrote 30 years ago in "California: The Great Exception."

Coming at the height of the postwar floodtide of immigration from the East, this book defined and encouraged the dual notion of California as an American dream and national laboratory. It is McWilliams' view that California has developed in isolation as almost a nation-state with vast resources, and that it has attracted the "risk-takers" from the East who were willing to experiment with new ideas and programs.

In the years since that book, the exceptionalism which McWilliams saw as setting a pathway for the nation has persisted in myriad ways: environ-mentalism, the free speech and antiwar movements, militant conservatism, judicial activism, personal politics, no-growth, rock music, long hair, drugs, surfing, the Hula Hoop, innovative social programs, Proposition 13. Often, the movement that catches the nation's eye is the direct reaction to the one that preceded it.

McWilliams believes that the cult, fads and "kooks" which many Americans associate with California are an inevitable byproduct of the state's obsession with experimentation, and will continue to be part of the California landscape. In California, as Sen. Alan Cranston (D-Calif.) once observed wryly, "The oranges are supposed to be on the ground and the people in the trees."

In the two decades before he left California, McWilliams wrote nine books on the land, people, history and political habits of the region. All of the books are exceptional in their own right and most of them are seminal. The most influential was "Factories in the Field," published within a few months of "The Grapes of Wrath" in 1939.

Those were the days when conspiracy theories were fashionable on the right wing of American politics rather than on the left, and there were those among the critics of both books who charged that McWilliams and John Steinbeck (Whom he admired but never met) were in cahoots.

"Factories in the Field" was the first extensively documented study of migratory labor in large-scale U.S. farming, starting with the importation of Chinese and Japanese labor in the 19th century. At the time it appeared, McWilliams was California's director of immigration and housing, trying to enforce a long-ignored law requiring minimal standards of health, food and housing in the state's farm labor camps.

"Of all the books I've read dealing with the history and struggles of California farm labor, 'Factories in the Field' is the best book for the ordinary guy," said Cesar Chavez many years later when he was beginning his own organizing efforts. "I like it because it tells this guy, among other things, how during the past 100 years a group of land-greedy growers with the support of the courts and the government grabbed the land, degraded the workers and beat and killed and harried them out of the land when they tried to organize and defend their rights."

McWilliams' favorite among his books is "North From Mexico," less well-known than the others but probably the most prescient of his works.

Written three decades ago, it accurately predicted the pattern of persistent Mexican immigration, legal and illegal, that today has created a nation within a nation in the American Southwest.

"French culture is indigenous in Quebec in much the same sense that Spanish culture is indigenous in New Mexico," McWilliams wrote at a time when there was no separatist movement in Quebec. "... There is a time factor and a space factor involved in both situations which is not to be found in the usual European immigrant 'problem' in America."

Today, McWilliams sees the growing Mexican immigration that alarms so many Anglos in the southwest as an opportunity rather than a threat.

"The Mexican immigration is to the advantage of the United States," he said in a reflective interview last week. "Mexico is a frontier country in terms of our relations with all the rest of Central and South America, and the immigration will help us to come to terms with it."

At 73, McWilliams is still trying to come to terms with the region and state he loves best, understands most fully and which he still considers his home. He remains fascinated by Los Angeles, where the bright, sharp bungalows of his middle age are faded by age and smog, and by the continuing experimentation of California.

"The California exceptionalism shows itself in odd ways," McWilliams says. "What people do with the dead has some meaning. When you fly over those old, old cemeteries in Brooklyn and Queens, there are nothing but gravemarkers. In California, what do they do? They throw the ashes away they scatter them over the ocean, they do all kinds of novel things with respect to this very ancient and very meaningful ritual of what you do with the dead. There's more experimentation here by far with this than any where else. I think that's sign."

For McWilliams, the signs are mostly positive. It is an odd contradiction of the man, that he prizes and promotes muckraking journalism of the kind that exposed the horrors of the farm labor camps and the excesses of the FBI, but that his books, articles and speeches invariably end in an optimistic expression that the world will become a better place in which to live.

Concluding "California: The Great Exception," McWilliams called upon the state to "put on knowledge with its power and adopt, as an official policy, the same generous openhandedness with which its magic mountains have showered benefits on those lucky people, the Californians."

In an era of lowered expectations and diminished generosity it is a view that Carey McWilliams will echo today in Union Station.