The Voting Rights Act may die in Congress next year at Republican hands, but in the redistricting battle here the act has become the party's ally in its bid to take control of Congress.

The act assures minorities of the opportunity of proportional representation, if possible, and because of the demographies of Texas, what may be good for blacks and Mexican Americans also may be good for Republicans.

"We didn't know it would turn out this way when they passed the Voting Rights Act, but it sure helps us now," one Republican strategist said.

This is because, by carving out predominantly black or Hispanic districts, Republicans would have a better chance to win the adjacent, heavily Anglo districts.

The legislature, facing a June 1 adjournment, is moving quickly on redistricting. On Monday, the house approved a plan that would give Republicans at least three new seats and threatens incumbent Democrats Jim Mattox of Dallas and William N. Patman, whose district includes Corpus Christi. A Senate plan, called by one legislative aide "an incumbents' map," would give Republicans two or three seats. The final map will be drawn in a House-Senate conference committee.

Democrats control both houses of the legislature, but the plan also must win approval of Republican Gov. Bill Clements and the Republican-controlled Justice Department in Washington. Ultimately, the Texas plan may end up in court under the Voting Rights Act.

Republicans are in good shape in Texas because of demographic changes in the past 10 years. Perhaps no other major state in the country has been as dramatically altered through population shifts as Texas.

Two great waves of migration swept through Texas in the 1970s. The first, from the North, deposited thousands of new, mostly white and conservative voters around the major cities of Houston and Dallas. The other wave, from the South, brought an enormous influx of Mexicans across the border into the Rio Grande Valley, and to a lesser extent all along the southern rim of the state.

The migrations helped raise the state's population by 27.1 percent and will enlarge the Texas congressional delegation by three seats in 1982, to 27, Mexican Americans, who in 1970 made up just 16.4 percent of the state, account for 21 percent of the population.

Republicans today hold just five of the 24 Texas congressional seats. The only real issue in redistricting is how many more they will get.

"If we don't get at least three more seats, the plan is going to be unacceptable," said one Republican, indicating the minimum that might be needed to avoid a veto by Clements.

Other Republicans believe the party can do even better, pointing to the strong vote they received in congressional elections in November. Republicans won 39.5 percent of the congressional vote, but that includes six races in which no Republican was entered. In the 18 contested races, Republicans received 48.9 percent of the votes.

Nancy Sinnott, executive director of the Republican Congressional Campaign Committee in Washington, said she believes Republicans could win up to 13 of the 27 Texas seats, given the right redistricting map and a continuation of the kind of well-financed, aggressive campaigning that worked last year.

The Texas Republican Party has produced such a map, and it is a work of genius, if not of political reality. It is not even formally under consideration in this Democratic bastion.

In the past, white conservatives have acted like Republicans but voted for Democrats. This is no longer true. In 1978, Clements was elected the first Republican governor since Reconstruction. Ronald Reagan won 55 percent of the vote in November.Both men drew well in many rural counties that have formed the bedrock of the Democratic power base here, and they swept the suburban counties around Houston and Dallas.

The political climate is changing so rapidly that people joke about Anglo Democrats, who have controlled politics here since Reconstruction, as an endangered species.

In part through redistricting, Republicans hope to extend their growing power to all levels of government, and that fact is just beginning to register on the disorganized Democrats, many of whom seem oblivious to the political earthquake rumbling through the state.

The Republican map is favorable to potential GOP candidates in part because it capitalizes on the growth of Hispanic power. The GOP map attempts to create five Mexican American districts and two black districts, including a new black district in Dallas that is the goal of a minority coalition in the south Dallas area.

"Any effort to better represent the black community in Dallas will end up helping us," said Wayne Thorburn, executive director of the state Republican Party. "And that is also the case of those south Texas districts."

Mexican Americans, who are well organized this year, want five congressional districts where their candidates would have a good chance of winning, and for the first time have the Voting Rights Act working on their behalf.

The Voting Rights Act was passed in 1965 to prohibit discrimination against black voters in the South. In 1975 Congress extended the act and expanded it to include "language minorities."

As a result, the law now covers Texas. It prohibits "dilution" of minority voters, which means it is illegal to divide artificially a bloc of minority voters between two predominantly white districts. Under the Voting Rights Act, a district must be at least 60 percent minority to be considered a "minority district."

The Mexican American coalition has drawn a map to provide five Hispanic districts, including the El Paso district represented by Democrat Richard C. White and the South Texas district held by Democrat Abraham Kazen Jr. Kazen would be most endangered by the new lines.

From the beginning, the Mexican American coalition has been preparing for a possible suit challenging the final map.

"Our primary concern is to create as many Hispanic districts as we can," said Joaquin G. Avila of the Mexican American Legal Defense and Education Fund. "The legislature's goal is to preserve incumbents and to preserve existing party strength."

Although Republicans made overtures to Mexican Americans early in the process, a potential alliance collapsed in part because Republicans were not willing to give assurances of support to extend the Voting Rights Act next year in Congress. And both black and Hispanic legislators are wary of being used by Republicans in the redistricting fight.

Nonetheless, Republicans have wrapped themselves in the rhetoric of act to better their cause.

Democratic Chairman Bob Slagle said, "There is a great deal of hypocrisy on the part of the Republican Party in using the Voting Rights Act. Republicans have been against the Voting Rights Act, they didn't vote for it, or for extending it."

"The law is the law," one Republican said. "It guarantees the rights of politicial minorities. We think the same principles ought to apply to the Republican Party."