In the oppressive midday sun, Judge Roy Barrera Jr. moves down the streets with the grace of a natural politician, his bone white sport coat showing no sign of surrender to the heat. On the short walk from the Bexar County Courthouse, where he serves as the youngest state district judge in Texas, to the little Mexican restaurant where he often eats lunch, his eyes skim the crowd for people who must be acknowledged, and his greeting is disarmingly genuine.

His father, Roy Barrera Sr., was the first Mexican-American secretary of state in Texas, and Roy Sr. has been involved in Democratic politics in San Antonio most of his adult life. For 28 years, Roy Jr. followed in his father's footsteps as a staunch Democrat and party-line voter each election day.

But last October, Roy Barrera Jr. made an important decision, for himself and for the political future of the Southwest. He became a Republican, trading in his Democratic heritage for a judgeship dangled by Republican Gov. Bill Clements. It was a clear-eyed decision that may have gambled away his whole political career, for in 1982 Barrera must run for election to his judgeship in this heavily Democratic county.

At 28, he is the front lines of the Republican Party here, at once the great brown GOP hope and perhaps the most impressive warning to the Democrats that they can no longer take the Hispanic vote for granted.

Politicians both regionally and nationally, assume that the politics of the 1980s will be shaped in a major way by the emergence of America's newest voting bloc: the Hispanics. Already 15 million strong, their votes could recast the electoral balance in the Republican West by adding a new base of solid Democrats. But that conventional wisdom is contradicted by the complicated reality of Hispanic politics.

Hispanics are not a monolithic ethnic group that will vote solidly Democratic as black Americans do. Nor are they like other new immigrant groups that charged the electorate early in this century. Hispanics are both old and new, both middle-class and poor. And their political loyalty is up for grabs.

Nowhere is the emergence of this group more vivid than in the Southwest, where in Texas, California, New Mexico, Arizona and Colorado 57 percent of the nation's Hispanics reside. And nowhere is their diversity more apparent nor their power more contradictory.

Hispanics in the Southwest are a community divided, with an established population long assimilated into Anglo society and a new immigrant class still pouring across the borders from Mexico. Mexican Americans here are great in numbers but small in voting power. They are both typical of an emerging ethnic group and atypical of American minorities. They are both progressive and conservative, staunchly Democratic but in sympathy with much of today's Republican philosophy.

In his campaign last year, Ronald Reagan talked of home, family, neighborhood and work. San Antonio advertising executive Lionel Sosa, a Republican, calls these "the cultural imperatives of Hispanics," and the message apparently registered. An estimated 30 percent of the Hispanics in Texas voted Republican last year, according to Republican pollster Lance V. Tarrance. The Southwest Voter Education Registration Project puts the figure at closer to 20 percent, but agrees that last fallhs Democratic vote by Hispanics was the lowest in history.

Mexican Americans, with some exceptions, tend to be pro-family, pro-growth, pro-military, pro-country, anti-crime, anti-drugs, loyal to the Catholic Church, hungry for advancement and increasingly middle class. Census figures show that 33 percent of Hispanic families have incomes of more than $20,000, compared to 26.7 percent of black families. Few ethnic groups are forming small businesses faster than Hispanics. Between 1972 and 1977, the number of Hispanic-owned businesses increased by 53 percent, according to the Census Bureau. That compares with a 12 percent increase in black-owned businesses and a 31 percent overall rise in minority-owned businesses.

Roy Barrera Jr. typifies that emerging middle class, and as he gobbles down his lunch at The Mexican Manhattan restaurant, the story of his political conversion unfolds.

He was practicing law in his father's firm, having settled in after more than four years in the district attorney's office (where, he says, he never lost a case by acquittal) when a group of local Republicans approached him about a judicial vacancy.

"They were concerned about my experience . . . and with my politics," he says between bites of enchaladas, tamales, rice and beans. "My politics had been with the Democratic Party for all 28 years of my life. The question was posed, if I was appointed by the governor, would I run, and if so, would I run as a Republican."

He knew there was no choice.

"It was far more important to me to be extended -- and to accept -- an opportunity to make a contribution to the people of this city and to be an example to the Young Hispanics of this country that there is a place for us in the political process, regardless of what political party you're affiliated with, and an example that there is room in the Republican Party and . . . that the Republican Party wants to extend to young qualified Hispanics the opportunity they extended to me.

"I put those factors even ahead of any realistic political aspirations that I may have entertained, knowing full well that by accepting the governor's appointment, I would unhesitatingly run as a Republican, and recognizing that in Bexar County I could well be cutting my own throat for countywide elective office in the future. . . .

"What was encouraging to me was when I laid the cards on the table with people in [the governor's office] about what I had done for the Democratic Party and what I had not done for the Republican Party, it didn't matter. I informed them I had never worked for the Republican Party . . . and had never supported a Republican candidate."

The day after his interview with Clements' appointments chief, Tobin Armstrong, Barrera was informed that he had the job. It was a good deal for both sides. "They never asked me where I stood on the issues," Barrera says.

Clements has been especially aggressive about seeking Mexican-American support. Outside the San Antonio City Hall stands an obelisk erected last May that is dedicated to the Hispanics in Texas and signed by the governor. Even Henry Cisneros, the newly elected Democratic mayor, admires the symbolic power of the monument. "I think Clements is making tremendous inroads in the Mexican-American community," he says.

Last winter, Clements actively supported a 31-year-old Hispanic in a special state Senate election in the Rio Grande Valley, even though a loyal Anglo GOP activist was running. Clements' candidate, Harvard Law School graduate Ricardo Hinojosa, eventually lost in a runoff, but not before Clements had frightened the Democrats in an area they have long taken for granted.

In 1979, Clements appointed a young Mexican American, Ed Prado, to a judicial vacancy in Bexar County. Prado had no political experience and no name identification and lost the 1980 election, but he has since been nominated to be U.S. attorney here.

"The Republicans are hungry for the Hispanic vote," Prado says. "The youth movement is the Republican Party. The Democratic Party has been old and stagnant, locally at least. . . . If I had started playing party politics in the Democratic Party, I might have had to wait for 20 years for this opportunity."

Roy Barrera, Ed Prado and Ricardo Hinojosa are typical of many young Mexican Americans today: well educated, well paid, fully integrated into society and more independent politically than their parents. They are the kind of Hispanics who give the Republicans hope that the Democratic hold on this minority voting bloc can be neutralized.

But they would have no political future in either party had it not been for the struggle of earlier generations of Mexican-American politicians who cleared the ground on which today's Hispanic vote stands.

That story is best told in San Antonio.

Mexicans dominated city politics here shortly after Texas gained its independence from Mexico, but by the turn of the 20th century, the Mexican population had dwindled and its influence locally was negligible. Two events led to their resurgence: one was the development of agriculture in south Texas and the resulting demand for cheap labor; the other was the Mexican revolution, which brought many middle-class Mexicans north to Texas. Many of them settled in San Antonio, where they soon established businesses, and among them was the Munguia family, whose children included the mother of Mayor Henry Cisneros.

It wasn't until after World War II, however, that Mexican Americans began to play a role in local politics. In the early 1950s, a reform movement called the Good Government League (GGL) took over the city, introducing a city manager system and putting together slates of candidates that dominated local elections. The GGL was careful to include at least one Mexican American on its slates in those early days, then added a black and finally a second Mexican American, giving minorities three positions on the nine-member city council.

But the Mexican Americans were hand-picked by the city establishment. "They were not truly representative of the Mexican American Unity Council. True victories were rare among community-based Mexican Americans, but one who succeeded and was a beacon to others was Rep. Henry B. Gonzalez (D-Tex.), who was elected to Congress in 1961.

Mexican Americans in San Antonio had a large enough portion of the population a decade ago to be a force in local politics, but they were not successful until they learned how to set up the machinery that turned numbers into effective politics.

In the 1970s, the pieces of a genuine Mexican-American political movement began to take shape. Out of the civil rights movement and the Great Society came Mexican Americans well trained in local affairs, experienced in running programs, ambitious and hard-driving, who turned to politics as their route to the top.

At the same time, a yound Mexican American named Ernest Cortez began to organize a neighborhood movement based on the model of Saul Alinsky's Back of the Yards group in Chicago. Called Communities Organized for Public Service (COPS) and built around the Catholic parisshes, Cortez' organization educated poor Mexican Americans about local politics, encouraged them to confront the Anglo establishment and eventually destroyed the Good Government League.

"The COPS organization is the very key to what happened here politically," says City Councilman Bernardo Eureste.

At the same time, the hierarchy of the Catholic Church turned Mexican American as politically active yound priests were supported by their leaders, especially Archbishop Patrick Flores, the first Mexican-American archbishop in the region, and today one of the most powerful men in San Antonio.

The Mexican American Legal Defense and Education Fund used the Voting Rights Act to help force the city to shift from at-large elections to single-member districts, paving the way in 1977 for the first minority-dominated city council in this century. And the Southwest Voter Education Registration Project undertook drives that increased registration from 40 percent in 1975 to more than 60 percent in 1980.

The capstone came in April, when San Antonio elected its first Mexican American mayor.

Cisneros' election was a sign that Hispanics are on the brink of political maturity in this country, but there remains one big obstacle to overcome, and this obstacle is the reason the Hispanic vote today is so volatile.

Despite systematic registration drives and even a record turnout last November, only about 30 percent of the Hispanic population participates in the political process. The 1980 census numbers, showing a 55 percent increase in Hispanics since 1970, mask a voting population that is far, far smaller.

About one-third of all Hispanics in the United States are ineligible to vote, either because they are not citizens or are too young. In El Paso, for example, Hispanics account for 62 percent of the city's population. But of the 265,819 Hispanics, an estimated 65,000 -- or 24.4 percent -- are registered aliens and thus ineligible to vote. Thousands more are children under the voting age.

Today's demograhpics make it impossible to typecast the Hispanic vote in the Southwest, for while the bulk of the population votes like other immigrants and identifies with the Democratic tradition, a major share of the active vote is the emerging middle class, which is increasingly drawn to Republican principles.

"The day the Democratic Party could take [the middle class] for granted is gone," says Alicia Chacon of El Paso, a former Democratic national cmmitteewoman.

One key to the future of the Hispanic vote, some Republicans believe, could be a new national policy on immigration. If President Reagan is able to naturalize the millions of Mexicans who live here legally or illegally and institute a guest worker program that is acceptable to many Mexican Americans -- a tall order -- the Republicans might even lock up a share of that vote for themselves.