Raul Besterio, the superintendent of schools here, wants one thing understood from the start. "I'm not against educating the children," he said. "But when somebody else comes to dinner, you can only add water to the soup so many times without losing the flavor."

He is speaking of consequences for his school district of the recent Supreme Court decision allowing the children of undocumented workers access to a free education.

From afar, the court fight looked like a classic case of a conservative Anglo establishment attempting to hold down a rising minority, but here on the U.S.-Mexican border, the reality is far more subtle.

In this overwhelmingly Mexican-American community, the issue is less one of racism than of nationalism and economics.

"Those most traumatized by the court's decision are Mexican Americans at the lower rung of the ladder," said State Sen. Hector Uribe, who represents the lower Rio Grande Valley. "They see themselves as hardworking taxpayers. They see themselves paying for the education of illegal aliens' children. And they see their jobs being taken away by these illegal aliens."

Here at the southern tip of Texas, in one of the fastest growing school districts in the state and the one perhaps most directly affected by the Supreme Court's decision, the issue in the long legal struggle is symptomatic of a much larger worry among Mexican Americans--that their modest economic progress will be eroded by a continuing influx of illegal immigrants from Mexico.

It also is a fight over who pays for the consequences of a national immigration policy that has allowed illegal immigration from Mexico to flourish. Here in the Rio Grande Valley, there is growing resentment among these Mexican Americans that they are being asked to pay for it personally.

A survey of Texas attitudes taken by the University of Texas' Institute for Constructive Capitalism found that among various demographic groups, Mexican Americans were the most likely to believe that illegal immigration was the most important problem facing Texas.

The survey also found that families with incomes of less than $6,000--a category that would include many of the Mexican-American families along the border--were most hostile to the creation of a guest worker program as a partial solution to the immigration problem.

Attitudes toward educating the children of undocumented workers are even more revealing. According to a poll by Southwest Polymetrics of Austin, Tex., 58 percent of those surveyed in Uribe's district opposed the right of a free education to such children, and the percentage of Mexican-American respondents was even higher.

Many people have scoffed at the protests coming from Texas over the court decision. The Supreme Court, in its ruling, overturned a Texas law that prohibited the children of illegal aliens from receiving free schooling. Texas was the only state in the country that had such a law.

Mayor Henry Cisneros of San Antonio said he sees no ill effects on his city from the decision, arguing that undocumented workers already contribute directly or indirectly to the support of schools through property tax payments or rent on their homes.

But in Brownsville, which bears a disproportionate burden because of its proxmity to Mexico, the problem is more real. "We will continue to do all we can for people," schools superintendent Besterio said, "But we ask for help. . . . The door is open now and there is no way to close it."

Even in Brownsville, the numbers do not appear overwhelming. In a school district of 29,000 students, Besterio said, there were 810 children of illegal aliens in attendance as of June 2.

There were also about 1,400 children of legal aliens.

No one is certain how many children come across the border from neighboring Matamoros to attend the Brownsville schools using phony American addresses.

Besterio says he has no way to predict how many more children there will be this fall as a result of the court decision. "It could be 800, it could be 8,000," he said. "Your guess is as good as mine."

The Brownsville district is one of the poorest in the state. "It's a matter of the poorest of the poor educating the even poorer," Uribe said.

The district's $57 million budget is straining to keep up with the growing numbers of students arriving each year. Besterio said the district is building a new classroom every 15 days this summer to meet the demand.

He estimates it costs $1,575 to educate a child, not counting construction costs, and adds that the continual arrival of illegal aliens is disruptive. "Every time a new one arrives, a teacher has to stop what he's doing to help," he said.

Besterio and Uribe are sensitive to charges that they were working against their own people by supporting the now unconstitutional Texas law.

"Think how I feel," Besterio said. "I'm a Mexican American. People say why are you against your people. Well, I'm an American. I was born and raised here. The federal government made me a Mexican American."

What Besterio is seeking now is help from Washington in the form of federal aid. So far there has been little response.

Rep. E (Kika) De La Garza (D-Tex.), has introduced legislation calling for a three-year program of aid to districts which have a high concentration of children of undocumented workers. A second day of hearings on it may be held in early August. In the Senate, Sen. John G. Tower (R-Tex.), is preparing his own bill to help alleviate the financial strain on districts like Brownsville's.

Ronald Reagan came through Brownsville during the 1980 presidential campaign, and Besterio buttonholed him to explain his view of the problem. In October, 1980, Besterio received a letter from the candidate.

"I am disturbed that the Carter administration has allowed this problem to become a burden to your district's taxpayers," Reagan wrote. "You have my assurance that if I am elected president of the United States, your problem will be carefully reviewed."

So far there is no sign of White House support. "It's hard to get people to listen to you," Uribe said. "Now that it's gotten nationwide attention, maybe it will [be different.] But I'm not very hopeful."