Jimmy Carter won 87 percent of the Hispanic vote here in Texas four years ago, and barely carried the state. With a Republican governor building party strength in the big cities and suburbs, Carter has to do even better with the Hispanics to win Texas this year.

But a visit to San Antonio, and a trip through the Rio Grande Valley with Ronald Reagan, suggest that the Democratic mission here is almost impossible. The Hispanics are on the move in just the ways that have caused so many other ethnic blocs to part company with the Democrats.

The great majority of the Mexican-Americans in Texas, to be sure, work as farm labor in the Valley or at low-paying jobs around the barrio on the west side of San Antonio. Virtually every amenity of life in the barrio -- paved streets, flood control, running water, housing projects and the new high school -- stem from federal aid programs.

Not surprisingly, most Hispanics here tend to identify improvement of their lot with the Democratic Party, which sponsored those programs. "We became Democrats," one Mexican-American put it to me, "in the same way we became Catholics."

But the man who made that remark is Ed Prado, an articulate 32-year-old lawyer who was named to a vacant judgeship by Gov. William Clements. Now Judge Prado is running for the job on the Republican ticket, and he introduced Ronald Reagan when the governor spoke in San Antonio the other day. Everybody, including Prado, understands that his name was put on the ballot as a magnet for Hispanic votes. But Prado shrugs his shoulders and asks what's wrong with putting the Republicans in debt to the Hispanic community. "The Democrats tend to take us for granted, and the time may come when the Republicans can do us some good."

A friend of his, and the man who guided me through the barrio, is an attractive 34-year-old urban economist Councilman Henry Cisneros, who is widely touted as a likely candidate to be first Hispanic mayor of San Antonio. Cisneros believes the city is on the verge of a great future. The trick is to plug the Hispanic community into the mainstream of Sun Belt expansion. So Cisneros has been working to bring new industry (Control Data and Levi Strauss, for example) and higher education (in particular, a graduate engineering school) to the city.

Development, according to Cisneros, can work for the Hispanic community only if it is associated with social purposes. He identifies these with the Democrats, so he backs Carter, but with reservations, and some doubts.

A city councilman who supports Carter strongly is Bernardo Eureste, a former backer of Sen. Edward Kennedy. Councilman Eureste has rallied to the president and is pushing his cause by stressing hard the issues of economic deprivation and racial discrimination.

But that approach puts off older Hispanic leaders who are used to making only modest demands of the Anglos. Moreover, there are some Hispanics who are clearly beginning to prosper in small business. Luis De Larosa, a 40-year-old realtor in Harlingen, a county seat in the Valley, is an example. He cast his first vote for John Kennedy in 1960 and was a member of the local Carter committee in 1976. He hs done well since then, in part because the Valley has become a popular vacation spot.

But this year he is on the Reagan committee, and he was in attendance when Reagan addressed a large and enthusiastic crowd in Harlingen. Why? Because," he told me, "if we didn't have such high interest rates and so much inflation, I would have done a lot better."

Those cases, of course, are highly personal. The drift they reflect could be reversed by some surge toward the Democrats set in motion through a big event, or even a whirlwind sockeroo visit from Sen. Kennedy. But that is precisely the point. The Republicans have set their sights on very limited goals -- pulling a few percentage points away from the normal Democratic vote. They brought Reagan to San Antonio and the Valley with that end in view. Though he is not personally very popular, he can realize that modest aim. But for the Democrats, however, it will take major development, a big surge, to run up the kind of Hispanic vote they will need to carry Texas.