Lucilla DeLeon has been a county commissioner here in Jim Wells County for 54 months. The first 48 she earned by winning a 1976 election. The last six she owes to the Justice Department.

For six years Jim Wells County, where Lyndon Johnson's famous 87-vote victory in 1948 propelled him into the Senate, has been unable to draft a redistricting plan that meets the terms of the federal Voting Rights Act.

Three times since 1975 the county has approved a redistricting plan. Three times the Justice Department has rejected it under the Voting Rights Act, saying it would hurt Mexican-Americans, who make up 67 percent of the population but have just one representative, DeLeon, on the commissioners court.

The four commissioners will try again Thursday night, and DeLeon hopes that this time she and her colleagues can agree on something.

"I think we may be able to draw a plan and satisfy at least three of us," she said the other day in her office on the south side of town. "Then we've got something going. But if they split up the [Mexican-Americans], then we'll go back at it again."

No one is overly optimistic -- and with good reason. The commissioners have been at this for more than six years.

The county has been in litigation with minority groups for most of the six years, and last November was forced to cancel its election rather than use the existing discriminatory precinct lines.

"Without the Voting Rights Act," said DeLeon, "we wouldn't have been able to succeed."

Among Mexican-Americans in the Southwest, no issue is more emotional than extension of the Voting Rights Act.

"Of all the things worth fighting about, this is the most important to Hispanics," said Henry Cisneros, the newly elected mayor of San Antonio. "They can take money and programs from you and you can survive. But if they take away your momentum, your leadership examples, the safeguards . . ., forget it. . . . It would be like chopping us off at the knees and then expecting us to run the race."

In the past few years, Mexican-Americans have made dramatic gains in political participation. Registration has risen substantially and turnout in the 1980 presidential election was the highest ever. But at the local and county level they are still struggling to gain a share of power, and many of them believe that, without the Voting Rights Act, their cause would be hopeless.

"If you didn't have Section 5 [which requires jurisdictions to get Justice Department approval before changing elections procedures], there would be a discriminatory plan in effect in Jim Wells County," said Joaquinn G. Avila, a lawyer with the Mexican American Legal Defense and Education Fund.

In July, a House Judiciary subcommittee will begin writing a bill to extend the act, the first step in a proces that will conclude in 1982, when most portions of the act expire.

President Reagan has not taken a position on the act, although he has asked Attorney General William French Smith to study it. The issue presents Reagan with a difficult political problem. Some conservatives would like to let the act expire; others would extend it nationwide (it now covers nine southern and southwestern states and numerous localities elsewhere) or weaken the preclearance requirements, which local officials argue are burdensome.

Texas was brought under the Voting Rights Act in 1975, when the act was expanded to cover language minorities. In the subsequent six years, Texas has received 128 letters of objection from the Justice Department -- more than any other state.

One reason for this record is the large number of political jurisdictions in Texas -- 16,000. But it also results from continuing acts of discrimination against black and especially Mexican-Americans voters in many rural counties.

On June 5, Avila testified before the House Judiciary subcommittee on civl and constitutional rights in Austin and presented strong evidence to back up his assertion that "discrimination is alive and well today in Texas." Among MALDEF's findings:

Of the state's 254 counties, 59 have not drawn new redistricting plans since the 1970 census; some have never done so.

In 51 counties with Mexican-American populations of more than 20 percent, there are no Hispanic representatives on the county commissioners court, the principal governing body.

In Edwards County, with a population of 2,000, one precinct had just 200 persons while another had 1,500, with most of the Mexican-Americans in the largest. When the county redrew its lines under pressure from MALDEF, Mexican-Americans were spread evenly among four precincts, with little hope of winning any of them. The Justic Department objected and a more equitable plan was adopted.

In Medina County, 43 percent Hispanic, the Justice Department rejected two proposed redistricting plans drawn under pressure from MALDEF, and the 1980 elections were canceled before a third plan was accepted.

In Frio County, a Mexican-American won the Democratic nomination for county tax assessor and collector in May, 1980. The county immediately proposed to shift the voter-registration responsibilities of that office to the county clerk.

The Southwest Voter Registration Education Project and the Texas Advisory Committee to the U.S. Civil Rights Commission also have documented evidence of discrimination against Mexican-Americans and the role the Voting Rights Act has played in resolving many of those disputes. The groups described harassment, ballot tampering, changing the location of polling places and refusal to comply with the basic procedures of the Voting Rights Act.

Here in Jim Wells County, about 40 miles west of Corpus Christi, T. L. Harville, county judge for 19 years, rejects assertions of discrimination. "There's nothing farther from the truth," he says. "There has been absolutely a sincere effort to comply with the law and satisfy the people here."

Harville says he will accept any plan the Justice Department will approve.

But his terms may make that impossible. He said he would not approve a plan that moves an incumbent out of his current district.

He further charged that Lucilla DeLeon is the roadblock because she wants all four commissioners to be Hispanic, and asserted that she has a brother in the Justic Department who is helping to block the plans sent to Washington from here.

"My brother is an FBI agent," DeLeon said. "He cannot interfere at all."

DeLeon also denied that she was holding out for four Mexican-American seats on the commissioners court.

"Not really, sir," she said. "All I want is an equal share of what rightfully is ours."