The Reagan administration has asked the Supreme Court to interpret the new Voting Rights Act narrowly and allow the city of Lockhart, Tex., to keep an at-large election system under which only one nonwhite holds a public office.

Although Lockhart's population is 55 percent black and Hispanic and 45 percent white, the City Council has only one minority member.

Laughlin McDonald, who handles voting rights cases for the American Civil Liberties Union, reacted to the Justice Department action in the Lockhart case, scheduled to be heard by the Supreme Court next week, by saying its interpretation of the Voting Rights Act violates the intent of Congress in changing the law last summer.

Lockhart is one of a number of voting districts with a history of racial discrimination that have been required to obtain Justice Department approval for any changes in local election law.

Lockhart changed its form of city government in 1973 without federal "pre-clearance," and four Mexican Americans filed suit in 1977 claiming that the new system violated their rights.

During the Carter administration, the Justice Department represented minority voters in the case and won in the federal district and appellate courts. But after President Reagan took office, the department changed its position and now has taken Lockhart's side in the brief it has submitted to the Supreme Court.

In documents filed last June at the Supreme Court, Justice Department officials argued that, even though the system in Lockhart may be discriminatory, the minority voters were not any worse off than before. This has been the legal criteria in similar cases, but lower courts had found that the minority voters were worse off after the change in Lockhart.

As a result of changes made in the Voting Rights Act by Congress last summer, any election system in the country can now be overturned if there is proof that it has the effect of discriminating against a minority group, even if the discrimination is unintentional.

Congress' report on the act also says that in any case that violates that part of the law, it is not necessary to prove a change in election procedures was retrogressive, making the affected minority group worse off than before the change.

After Attorney General William French Smith and William Bradford Reynolds, head of the department's Civil Rights Division, tried unsuccessfully to argue Congress out of enacting the new version of the Voting Rights Act, the administration agreed to a compromise version.

But Reynolds this week, in his new brief to the Supreme Court, arged that the retrogression standard in the old law is still applicable because Lockhart was originally brought under the preclearance section of that act, not the part amended by Congress. He said the Supreme Court should side with Lockhart and send the case back to the trial court.

Some lawyers in the Justice Department's Civil Rights Division appear to have been surprised by Reynolds' interpretation.

In a U.S. District Court brief filed Oct. 18 in a voting rights case in Sumter County, S.C., the Civil Rights Division noted that "Congress has made it clear" that a local jurisdiction's election plan cannot be cleared by the Justice Department if it would violate the section of the law covering the whole country.

The brief was abruptly retracted, and a new one filed this week, with the offensive language deleted.