As Mexican-Americans struggle to achieve full political and economic equality in the Southwest settled by their forebearers, they frequently see themselves as foreigners in their own land.

In a region where the Spanish language and the Mexican culture are apt to be regarded as a badge both of separateness and inferiority, Mexican-Americans seek to become full and active citizens while simultaneously preserving their heritage, their way of life and their church. Sometimes the strain of trying to accomplish all this seems too much to bear.

"There is still oppression and discrimination," cried Mario Obledo, a California state government official at a recent Mexican-American rally in Los Angeles. "What happened to our pride, our spirit, our unity, our inner selves? Why did we give up so easily? What happened to our culture and our very being?

Obledo's words touched the hearts of an audience that knew that its Spanish, Mexican and Indian ancestors ruled a half million square miles of the Southwest - all of the present states of California, Arizona, New Mexico, Utah and Colorado and most of Texas - for three centuries before it was taken away from them by force of arms.

Sometimes this knowledge is expressed defiantly, as it was this same evening by Rep. Edward R. Roybal of Los Angeles, one of four Mexican-Americans in the House of Representatives.

"Our roots were planted in the soil of the United States long before they [the Anglo-Americans] arrived," Roybal said. "Our boys fought on the battlefields of Europe and Korea and Vietnam - and we intend to remain here."

Roybal's speech was a reminder that MexAmerica is a phenomenon both old and new.Before the Southwest was American it was Spanish and; after that, Mexican. But for the first time since the Treaty of Guadalupe Hildalgo in 1848, Mexican-Americans are becoming an influential minority within the borders of the United States. This minority is the largest in the No. 1 and 3 most populous states, California and Texas, and with the help of other Hispanic immigrants from the Caribbean islands and South America, soon will attain this status in the entire country.

Indeed, there are those who say that the Mexican-Americans will be able to reconquer by high birth rate and unceasing immigration what they lost on the battlefield more than a century ago.

All migratory waves arouse fearful nativist feelings in the United States. "Old-stock Americans have become restless," warned a journal in 1924. "They are dissatisfied with the denationalizing forces at work in this country. There is something wrong, and the American people know there is something wrong . . ."

These words are from a Ku Klux Klan magazine, Fiery Cross. They were the prelude to an anti-Catholic article in which the author worried that-Catholics, Jews and blacks would make common cause against native white Americans. Now, crosses occasionally burn again on California hillsides, and respectable people warn against being "denationalized", by bilingual education.

The Klan still is small potatoes in the West. Mexican-Americans who have never seen the burning crosses are reminded in subtle ways - and sometimes in ways not subtle at all - that they are not fully accepted citizens in the United States.

The car of a distinguished-looking Mexican-American businessman was halted at the Immigration and Naturalization Service's San Clemente, Calif., checkpoint, and the Los Angeles-born industrialist was interrogated about his citizenship while cars full of unkempt Anglo surfers heading to the beach were waved by without being stopped.

Such humiliations remind Mexican-Americans that they are suspended like tightrope walkers between two nations and two cultures. If they go to Mexico, they are apt to be regarded as "gringos." Some, like the southern California described by Jose Antonio Villerreal in the novel "Pocho," express contempt for Anglo-Americans and hauteur toward Mexicans, becoming "truly a lost race."

With history on their side, Mexican-Americans are apt to argue that they are different from other immigrants to the United States. As the continent's most senior immigrants, they say they are "Americans" - in the sense that Jefferson or Monroe used the word to describe all North Americans - as fully as the Anglos who came after them. And yet Mexican-Americans face many of the same problems as the earlier European immigrants and, like them, have responded in varying ways.

Some have assimilated while others have retreated into their culture. Some have sought to "be like everyone else" while others have proclaimed their heritage with pride. Some have protested and others have gone along with the inequities, hoping that time would blur the differences and the discrimination. And some, perhaps the majority, have attempted to assimilate and to keep their culture at the same time.

The old "melting pot" ideal of the United States fell on hard times in the ethnic 1970s. But don't try to tell that to the thousands of Mexican-Americans who live side by side with Anglo-Americans in middle-class communities in San Antonio, Albuquerque, Phoenix and Los Angeles. Don't try to tell it to the tens of thousands of Mexican-Americans in California whom Ronald Reagan in 1966 found so receptive to his conservative message that he took to ending speeches with the Spanish phrase, "Ya basta!" Its rough equivalent in English: "Had enough?"

One thing that Mexican-Americans of any political persuasion have not had enough of is respect for their own cultural heritage from the Anglo-American majority.

"I don't believe we're going to fall into the melting pot," says Democrat Alfredo Guiterrez, the state Senate majority leader in Arizona. "All we're going to fight for is a piece of the pie but in a pluralistic society. We want to keep our culture and our language so that we can survive culturally in the United States."

Vilma Martinez, general counsel of the Mexican-American Legal Defense and Educational Fund, believes that national leaders have a 'black-white orientation," reflecting a lack of national understanding or interest in Mexican-Americans.

"Look at television, for God's sake," says Martinez. "The first program about a Mexican-American was Chico, and then they hired a Puerto a Puerto Rican-Hungarian to play Chico, and then they hired a Puerto Rican artist to write the music. I like Jose Feliciano, but that's not Chicano music."

Martinez thinks that Mexican-Americans suffer because they are coming of age politically "not in the Age of Aquarius but the age of [Allan] Bakke." She means they are clamoring for full civil rights at the very time that the tide is running against civil-rights protects by minorities.

Whether or not their timing is right politically, Mexican-Americans have a better chance than other immigrants to preserve their cultural heritage. Both the Mexican-American culture and the Spanish language are constantly replenished in the Southwest by ndew arrivals from Mexico who speak little or no English.

Unlike many of the European immigrants, the Mexican migrates into an area related to his history and experience. Much of the Southwest is physically similar to northern Mexico - hot, dry and barren with vast expanses of mesas, mountains and mesquite. The place names are Spanish. For many Mexicans, it is more like moving to a richer neighborhood than migrating to another country.

Nonetheless, Mexican-Americans experience the same generational conflicts and inner-group prejudices as other groups. "The younger ones sometimes are embarrassed by their elders," said Jaime Rodriguez, a historian at the University of California-Irvine. "They don't think they dress right or speak well."

Among Mexican-Americans - but rarely in the presence of Anglos - a crude and unlettered person is sometimes called "a Tijay," deriving from the Mexican border city of Tijuana. It would be the equivalent of saying of a European immigrant of an earlier day that he just got off the boat.

But a strong case could be made that even the newest Mexican arrivals in the Southwest are a stabilizing and socially beneficial addition; particularly in a southern California associated with hedonism and a disintegrating family structure.

Despite upsurges of nationalism, the Mexican-American's greatest attachment has been to his home, his family and his church - hardly harbingers of revolution. At the same time, Mexican-Americans and others from Latin America have constituted a major political influence within the Roman Catholic Church, one institution already reacting strongly to the rise of MexAmerica. In 1970 there were no Spanish-surnamed bishops in the region. Now there are six.

Much of the pressure to enlist the prestige of the church in behalf of campaigns for social justice has come from the Padres a southwestern association of activist Hispanic priests. These priests find their voice in leaders such as Bishop Patrick Flores of San Antonio and Bishop Juan Arzube of Los Angeles. Arzube, an Ecuadorian, has been an exponent of the so-called "liberation theology" developed in Latin American countries, which combines religious preachings with socio-political action.

Father Juan Romero is one of the Padres. He comes from a New Mexican family that traces its ancestry back to the 17th-Century Spaniards. Now he is a Los Angeles parish priest who, with others, spent 14 years trying to convince their superiors to let them serve the people in their own language.

"I would sit in the confessional hearing the people complain in Spanish, but we had to hold Masses in English," Romero remembers. "To some of the pastors, teaching English was more important than teaching the gospel. It was tearing us up."

Romero seems to be struggling toward a synthesis between the seemingly conflicting poles of assimilation and ethnic identity. He would understand the words of Thomas Wolfe, who wrote: "I think the true discovery of America is before us. I think the true fulfillment of our spirit, of our people, of our mighty and immortal land is yet to come."

This fulfillment is approaching in MexAmerica! "We have come to an equilibrium by understanding who we are - Mestizos ('mixed-blood") and Americans, something different, says Romero. "We are a new people that has a destiny. It's a sense of peoplehood that is just now coming. . . We've been here longer than anyone else. We belong here."

Staff writer Bill Curry and special correspondent Bruce Cory contributed to this report.