In the bustling super-mercado on Fulton Street, the cash registers work hard as shoppers check out their goods. Of every $10 rung up at the Fiesta Mart, the owner estimates, as much as $4 comes from the pockets and purses of illegal aliens.

So many illegals have swollen the population in Houston's east side barrio, in fact, that many businesses are prospering, and the Fiesta Mart is even building another store.

In Fresno, Calif., as in most of that state, the lush farms of tomatoes, cantaloupes, vegetables and specialty crops seem to require more field workers than America can supply. "Everybody knows," said one farm industry man, "we're using Mexican labor (illegally) to harvest the crops."

Along the 1,933-mile border the United States shares with Mexico, Mexicans daily cross over to shop at better-stocked American stores or pick up their mail at a U.S. post office, where the delivery is faster.

Poor American women cross to Mexican border towns for cheap abortions, businessmen to talk deals over true Mexican food, and the U.S.Postal Service warns cost-conscious businessmen not to make mass mailings from across the border, where postage is cheaper.

This "blurring of the border," a routine reality of daily life, last week received a startlingly candid and high level recognition with the disclosure of a secret White House document outlining American's choices in its future foreign policy with Mexico.

"No decision of any consequence on either side of the border can have purely domestic or purely foreign repercussions," said the document, known as Presidential Review Memorandum (PRM) 41. "Migration and economics are increasingly linking the politics of the U.S. and Mexico, and not only in the Southwest border communities."

For across the Southwest, in places as far from the border as Los Angeles, Houston and Denver, the people, culture, commerce and language of two countries have mixed so extensively that they have produced a unique hybrid, a virtual nation within a nation-MexAmerica.

Now with the disclosure of the White House paper, MexAmerica has emerged, like a true nation, to both influence and feel the impact of foreign policy.

Perhaps never before in peacetime has a U.S. foreign policy decision reached so deeply into the lives, livelihoods and neighborhoods of Americans as this one will reach into MexAmerica.

More than 7 million Mexican Americans live in the five states of MexAmerica, and there are untabulated millions more Mexicans living here outside the legal system of quotas and work permits. Much of the agriculture in the same as in Mexico, and businesses on one side of the border draw customers, cash and sometimes co-workers from the other.

Thus, in MexAmerica, employment and business activity, farms and factories, schools and social services, wages and welfare will be affected acutely by whatever policy decisions the United States makes with Mexico on worker migration, sales of farm produce ant the openness of the border.

So in a peculiar sense the U.S. foreign policy now being shaped toward Mexico will be as well a "foreign" policy toward MexAmerica-California, Texas, New Mexico, Arizona and Colorado.

PRM 41 maps out the pathways the United States can follow during coming years in its dealings with its poor, underdeveloped neighbor. It was prompted in September by Mexico's increasing discoveries of rich, black crude oil.

For the United States, that oil, and natural gas, could represent a closer, more reliable source of energy, one less subject to the skipping political fires of the Middle East.

For Mexico, it could represent new wealth to address its social problems and a new enticement to gain from its rich, powerful neighbor concessions on aid, agricultural sales and increased immigration to the United States.

Migration is, for now, Mexico's safety valve, a gradual release of the pressure of unbelievable joblessness and a population growing so rapidly it will double to 130 million by the turn of the century.

PRM 41 says one course for the United States would be to treat Mexico as it treats many other countries, according it no special treatment. Another option would be to treat Mexico as a partner and extend to it such special considerations as more employment for Mexican nationals in the United States.

However, here in MexAmerica-where the Mexican government has sent Spanish-language textbooks to a poor south Texas school system and where the United States has collected tens of thousands of dollars in back wages on behalf of Mexicans who were underpaid while working illegally in Texas-the option of partnership is viewed not as a choice but as an affirmation of what has happened.

"It's clear that all employers and most unions and public officials are simply unwilling to say publicly what is the truth, which is the these (illegal) workers are needed because domestic unemployed won't do the job," said Jerry Lopez, a UCLA law professor who is preparing an analysis of immigration policy.

"The foreign policy considerations will expose to the American people what is actually happening."

Widespread estimates, for example, put the use of Mexican labor in California at a level exceeding that of the legal "bracero" program that ended in 1964.

Growers and some public officials say the harvesting of many of California's 200 crops could not be accomplished without those workers because they do work Americans don't want at field wages. And the Mexicans sometimes earn more money in two or three weeks of field work in the United States than they do at home in a year.

Whatever their value, these workers often are subjected to exploitation, harassment and the fear that accompanies their shadowy existence. Much of that could be reduced if, as PRM 41 suggests, increased migration of workers were regulated and matched to U.S. labor needs. That is, made legal.

Again, that is the increasing reality, More and more, the alien who lacks a work permit permit is finding protections, rights and privileges being extended to him and his family-the "blurring of the border" as PRM 41 puts it-whether he is in the melon fields, garment factory, or the landscape crew.

He is being organized into labor unions, his children are getting free public education (sometimes by court order, sometimes by state policy) and his wages are being protected. In one case, a Texas prosecutor moved in on an employer who would turn in his illegals just before they were to be paid.

Whatever these realities, any decision on future relations with Mexico will be marked by conflict and contradiction. Labor feels illegal immigration supresses wages and takes jobs from americans. Many farmers want to allow cheap Mexican labor into the country while keeping out competing Mexican farm products.

"The west coast of Mexico is a major competitor to our winter vegetables." said Les Hubbard of the Western Growers Association. If the United States relaxes trade barriers "during periods of time when we're flush and can be competitive, that would be okay," Hubbard said. "By reducing tariffs in the winter, they would simply be giving our livelihood away."

Harry Kubo, president of the NISEI Farmers League, said Mexican farm products undercut American prices because of low wages in Mexico and high U.S. costs for labor and refulations covering the environment and health.*tWhatever course President Carter eventually sets for U.S. relations with Mexico, the reality of partnership and interdependence in MexAmerica is likely to continue, short of absolutely sealing the border.

Even then, Hispanics-already the largest minority in MexAmerica - will be within a decade the larges tminority group nationwide, and as PRM 41 notes, Mexican Americans "are the backbone of our important Hispanic minority. Many of their needs and problems involve Mexico."

Now, 130 years after America wrested much of MexAmerica from Mexico, some Hispanic leaders hope the current review of relations toward Mexico may help bring to an end the long years of discrimination against and disdain for Hispanics.

As the review memorandum puts it: "Relations with Mexico pose fundamental questions for the United States. Their evolution over the next generation will influence what many Americans do for a living, even who Americans are and how they treat each other."