A nation within the nation is emerging in the South-west.

Its language is a hybrid of English and Spanish. Its culture is a blend of modern, technological United States and developing but still rural Mexico. Its existence is most evident along the 1,933-mile border that the United States shares with Mexico, but it is highly visible as well in such diverse nonborder cities as Los Angeles, Phoenix, Alburquerque, Houston and Denver. Its existence poses a threat to the American melting pot ideal greater than ever faced from the Irish, the Czechs, the Italians and the Poles.

Its name is MexAmerica, and the lessons it is teaching the larger nation are not limited to the Southwest.

By the mid-1980s, the number of Hispanic-Americans - including immigrants from Caribbean islands and South America as well as Mexico - is expected to exceed the 30 million projected for American blacks.

Today, in the southwestern states of Texas, Colorado, New Mexico, Arizona, and California, Mexican-Americans already vastly outnumber blacks, Asians and all other minorities, reaching as high as 36 percent.

They even outnumber "Anglos" in many of the fast-growing Sunbelt cities that dot the hot, dry, mesquite-covered landscape from San Diego to Brownsville.

"A binational, bicultural, bilingual regional complex or entity is emerging in the borderlands," says historian and retired National editor Carey McWilliams. "Nothing quite like this zone of interlocking economic, social and cultural interests can be found along any other border of comparable length in the world."

The history of MexAmerica dates back to the conquistadores and mission padres who roamed the area that is now the American Southwest. But its startling growth, both in numbers and influence, is a recent phenomenom. The Mexican component of this two-nation society continues to boom on both sides of the border. Mexico itself has a higher birth rate than Bangledesh.(See MEXAMERICA, A10, Col.6)(MEXAMERICA, From A1)

In the United States, the Mexican-American population, which may have been severely undercounted in the 1970 census, is growing steadily and is believed to total more than 7 million in the Southwest alone.

Even in Diboll, Tex., in the Dixie-oriented pinewoods section of the state near Louisiana, Mexican-Americans are nearly numerically equal to blacks, and the town's largest employer, plywood company Temple Eastex, is considering instructing new employes through bilingual film strips.

Los Angeles, with a larger population of Mexican heritage than any other city except Mexico City, is considered the capital of Mex-America. It is home to 1.5 million citizens of Mexican ancestry and perhaps 500,000 more illegal immigrants. Large sections of east Los Angeles and the downtown area, sometimes derisively called "Baja Hollywood," are indistinguishable from similar areas in large Latin American city. The language that is spoken there is both Spanish and English and often, as in the phrase, "presta mi su credit card," it is a mixture of both languages that irritates purists of either one.

For the first time, Mexican-Americans outnumber either Anglo-Americans or blacks in the Los Angeles school system. The world that will be emerges most clearly in kindergarten where 50 percent of the children claim Spanish as their first language. And the Los Angeles police force for the first time is requiring all of its cadets to take six months of conversational Spanish.

In E paso, Enrique Perez grew up when public school students were detained after school if teachers overheard them speaking Spanish during school hours. Today, Perez is the school system's director of federal programs, which helps fund a $5.5 million bilingual education program that teaches Spanish to Anglos and English to Mexican-Americans with the goal of making students fluent in both.

In New Mexico, Jerry Apodaca recalls the days 35 years ago when he lived with his family in across-the-tracks segregated housing in Tyler, Tex., where his father was a soldier. Today, Apodaca is the first Mexican-American governor of New Mexico since 1917. Ineligible to succed himself under state law, he is looking forward to running for the U.S. Senate in 1982.

And in Arizona, Mexican-Americans last week celebrated Good Friday as it was celebrated in Mexico City - by visiting special altars at seven different churches. For the past four years in Phoenix - and for the past 19 in Tuscon, nearer the border - well-off Mexican-Americans have adopted the Mexican custom, following a tradition of 16th-century Spain, of presenting their 15-year-old daughters to society at events known as Quinceanera balls. At the other end of the socioeconomic spectrum, the Mexican-American sections of Phoenix are organized into 16 barrios - or neighborhoods - where residents warn each other of approaching welfare workers or policemen.

The growing Mexican influence is seen in food, fashion, architecture and music.

The Mexican milk candy, dulce deleche, is now sold outside the barrios. So are the Mexican embroidered dresses favored by many Anglo women during the long, hot southwestern summers. Tex-Mex fast food stands and cafes, doling out tacos and buritos with hot sauce that many Mexicans regard as barbarously American, abound.

Department stores in the Southwest feature racks of "disco Mexicanos," long-playing records of Freddy Fender singing in English and Spanish and of such Spanish-languager favorites as Julio Iglesias, Silvestre Vargas, Pedro Infante and the Los Humildes 4.

Increasingly in MexAmerica, Spanish is the language of the airwaves. The Southwest used to have only a handful of Spanish-language radio stations. Now there are 37 in Texas, 28 in California, six in Arizona, four in New Mexico.

In Midland, Tex., cable television brings in the Spanish-language channel from San Antonio-based Channel 34 serves an audience of 2 million from San Diego to San Francisco with all-day broadcasting of news, variety shows and movies. At 7 p.m. daily the news-oriented Mexican-American customarily sits down to watch anchorman Javier Calodosky, known as the Walter Cronkite of Mexico, over the Spanish Information Network.

The written word in the Southwest also is becoming both English and Spanish. Popular magazines such as "Nuestro - The Magazine for Latinos" are written in both languages. So are emergency warning cards on Texas International Airlines, legal advertisements in Houston and dialing instructions on telephone booths throughout southern Caifornia.

Spanish is spoken by two-thirds of Catholics in MexAmerica Churches where the pictures of past priests named O'Reilly of Murphy adorn the parish office walls are served now by priests named Sanchez or Gonzales. Bishop Juan Arzube of Los Angeles, a leader in the growing "Latino" movement in the church, regards the Catholic parish as the basic building block for organizing Mexican-Americans whom he sees as largely unmeltable ethnic group.

Unmeltable they well may be. In the 1960s third-generation Mexican-Americans experienced a rebirth in pride of heritage not dissimilar to that by third-generation Americans whose grandparents came to the United States from Europe.

They called themselves "Chicanos" appropriating what used to be a neutral term used to describe Mexicans who lived in the United States. (Many people of Mexican heritage who lived in the Southwest at the time of World War II referred to themselves as "Mexican." The term "Mexican- American" came into usage at the time of that war, in which Mexican-Americans fought in disproportionately high numbers.)

But in the 60s "Chicano" became a proud badge of ethnic identification among the young, as "Latino" has become popular in the '70s.

The metamorphosis of self-image has continued because the cultural influence from south of the border has been continually renewed. Unlike European immigrants, who were separated from their roots by an ocean, they are separated from Mexico only by a common, indistinguishable border.

The influence of the United States also is strong in Mexico, particularly in the interdependent border region. While Spanish is taught in the high schools and colleges of the U.S. Southwest, English uniformly is offered in Mexican schools. Clusters of twin factories known as maquiladoras dot the border region, taking advantage of cheap Mexican labor and a custom-free zone to hand-finish radios, toys and calculators.

American foodstuffs and clothes are both necessities and status symbols in Mexico, which is the fourth-largest customer for U.S. exports.

In the border region a crisis in one country frequently means a crisis - or an opportunity - in another. Unemployment in rural Mexico drives illegal immigrants north. The Arab oil embargo brings American motorists into Mexico to buy its plentiful gasoline. When the Mexican peso devalued in 1976, business on the American side of the border slumped so severely that Texas Gov. Dolph Briscoe asked the Small Business Administration to declare El Paso and five border counties a disaster area.

The man who epitomizes the two-nation quality south of the border is Roberto de la Madrid, a charismatic bilingual politician who rose from shoeshiner to banker to governor of Baja California Norte. He once served as vice chairman of the San Diego planning Commission, and in the opinion of Lucy Killea, who runs the San Diego-based border organization known as Fronteras de las Californias, "could have as easily been elected in this country as he is in Mexico."

Indeed, de la Madrid and his young, aggressive staff favor Americans' political techniques and admire families such as the Kennedys. De la Madrid campaign posters said simply "Roberto." As governor, he has introduced daily press briefings and televised reports to the people.

Every few weeks de la Madrid flies his plane north for a meeting with Gov. Edmund G. (Jerry) Brown Jr., who comes down from Sacramento to meet him at a quiet Hollywood restaurant called El Adobe. It is a meeting of equals between two canny politicians who supporters believe will one day become presidents of their respective countries. The equality is based on mutual need: Brown wants Mexican oil and natural gas commitments as backstops for his antinuclear energy stand in California and de la Madrid wants American tourist dollars and a second border crossing at San Diego to bolster Baja's economy.

"We're inextricably linked with those people, and the sooner we realize it the better," Brown said in a recent interview. "Mexico's not an island. If something goes wrong in Mexico City, it will be felt in Los Angeles and El Paso."

Recently Brown demonstrated his political grasp of the growing importance of MexAmerican when he kept hundreds at a Democratic convention waiting for him until he addressed non-partisan Mexican solidarity rally in Los Angeles that concluded, "Viva la Raza (the Mexican people)! Viva yourselves."

The same day that Brown was making points with Mexican-Americans the state convention of the California Republican Party was passing a resolution condemning bilingual education. On its face this was an act of political folly demonstrating anew why the GOP has become a seemingly permanent political minority. But the action also reflected two other facets of MexAmerica - the longstanding political impotence of Mexican-Americans and the fear of some members of the Anglo majority that a new ethnic awakening is about to occur.

The situation is somewhat different in New mexico, where a Spanish-American tradition developed before English immigrants landed at Plymouth Rock. In other states, however, Mexican-Americans have never been represented in proportion to their numbers. They are about to become a political majority in San Antonio because of federal pressure to change the election system, but there is not a single Mexican-American councilman in such strongholds of the Mexican heritage as Los Angeles.

There are many in the Southwest who think that this historic pattern of political underrepresentation is about to change. They see protests and political stirrings in Texas, Colorado, Arizonia, New Mexico and California as signs that Mexican-Americans are on the verge of pressing as strongly as black people in the United States have done during the past generation.

This awakening is exciting to people such as Ramon Edardo Ruiz, a Latin-American historian who returned to the University of California at San Diego from New England - which he regards as "the only truly civilized place in the United States" - because he wanted to be around Mexicans and Mexican-Americans.

Ruiz believes that the Southwest has "everything to gain and nothing to lose" from the cultural infusion, but some Anglo-Americans worry that MexAmerica could in time become a pro-separatist Catalonia or Quebec.

This is a prospect generally discounted by scholars and Mexican-American politicians who instead envision a pluralistic, Third World society with heavy concentration of blacks and Asians, a society in which every ethnic group could be a minority.

That such a society may be well advanced in California was demonstrated earlier this month when the state Public Utilities Commision ordered telephone companies to provide statewide emergency service in Spanish. But the commission also ordered that this service be provided in Cantonese in the San Francisco area.

Today, in California, there are bilingual language associations for Spanish, Cantonese, Japanese, Tagalog (Filipino) Thai and Portuguese.

A study commissioned by California Lt. Gov. Mervyn Dymally, a black, predicted that California would be a Third-World society by 1990. In this society Anglo-Americans would be the largest minority, but the numerical proportion of Mexican-Americans would increase dramatically.

"There was a time when the white men came in and overran us," says Miguel Garcia, a militant Los Angeles lawyer. "Now it's like history in reverse."

The underrepresentation of the Mexican-Americans in the political system reflects an even greater underrepresentation in such professions as law and medicine. That is changing, too, though less swiftly than Mexican-Americans would like.

Ralph Ochoa, assistant to California State Assembly Speaker Leo McCarthy, remembers being the only Mexican-American to graduate from an accredited law school in California in 1969. Even today, Mexican-Americans tend to celebrate singular break-throughs: There is one Mexican-American regent of the University of California, one Mexican-American chancellor in the state university system, one Mexican-American cabinet official in state government.

As Mexican-Americans struggle to gain full equality, Bishop Arzube calls upon them to "upsurge like the Irish did."

"After all, we all celebrate St. Patrick's Day," said Arzube. "There will be a fear that we will dominate, as we might do for a while, but in the long run we just hope that the American culture will grow so it isn't Anglo-American but multicultural. That's the way it should be."

Echoing this idea in the secular political arena is California Assemblyman Peter F.Chacon, who made common cause with Asians to push through bilingual education programs over the opposition of some whites and some blacks.

"California, which always has been in the forefront, is going to show the vitality of a multilingual, multiculatural society," Chacon says. "This will be a real metropolitan state where there will be engendered a real respect for differences. it's an exciting prospect."

Washington Post staff writer Bill Curry, special correspondents Joel Kotkinand Richard Morin, and researchers Juliette McGrew and Kathy Dillon contributed tothis article.

NEXT: Economics of immigration.