The Supreme Court agreed yesterday to decide whether a key provision of the federal Voting Rights Act applies to election of judges, a ruling that could reshape the racial composition of state judiciaries throughout the country.

The court said it would hear cases from Texas and Louisiana in which minority voters have challenged parts of the states' judicial election systems, arguing that they result in dilution of minority voting strength.

The 5th U.S. Circuit Court of Appeals, in the cases the Supreme Court agreed to review yesterday, emphasized that Section 2 of the Voting Rights Act protects the rights of minorities "to elect representatives of their choice." Splitting 7 to 6, the appeals court said judges are not considered "representatives" under the law.

The court took the unusual step of moving up its announcement of the vote to hear the cases, which normally would have come on Tuesday. Spokeswoman Toni House said the justices acted to give the parties in the case more time to get ready for arguments in April.

Civil rights groups were delighted with the news. "Throughout the country state court judiciaries are populated by and large by white people," said Robert B. McDuff of the Lawyers' Committee for Civil Rights Under Law. "It's one of the remaining bastions of racial segregation in American government."

According to the Justice Department, 41 states elect some or all of their judges.

In the Texas cases, League of United Latin American Citizens v. Mattox and Houston Lawyers' Association v. Mattox, black and Hispanic citizens challenged the at-large method of electing district judges in a number of the state's metropolitan counties.

They pointed out that in Harris County, the state's largest, blacks made up 18 percent of the voting-age population, while three of the 59 district judges, or 5 percent, were black.

A federal judge found that the at-large system discriminated against minority voters. The 5th Circuit overruled him, saying the Voting Rights Act did not apply.

In asking the high court to review that ruling, lawyers for the minority voters argued that the appeals court's interpretation "will deny thousands of minority voters an equal opportunity to vote for judges of their choice in an election system free of discriminatory elements."

If allowed to stand, the lawyers said, "the law will be that discrimination in voting will not be tolerated, except in the election of judges."

Texas Attorney General Jim Mattox said the 5th Circuit ruling was right but agreed that the question in the case was "of undoubted significance to the nation's jurisprudence, many of its state judicial systems and minority voters."

The court also agreed to hear three cases from Louisiana. Two, Chisom v. Roemer and U.S. v. Roemer, concern the state's method for electing justices to its seven-member Supreme Court. Five of those justices are elected from single-member districts, but two are elected at-large from a sixth district.

Black voters in that district filed suit in September 1986, alleging that the system diluted black voting strength in violation of the Voting Rights Act. It said if the two-justice district were divided along parish lines, one district -- Orleans Parish -- would have a majority of black voters and a shot at electing a black justice.

No black has been elected to the Louisiana Supreme Court this century.

The 5th Circuit ruling in the Texas case resulted in the Louisiana cases being thrown out as well.

In asking the high court to hear the Louisiana cases, Solicitor General Kenneth W. Starr said the court had carved "a significant category of elections from the broadly worded text" of the Voting Rights law.

Lawyers for Louisiana Gov. Buddy Roemer (D) argued that considering judges to be "representatives" would "do violence to {and perhaps destroy} the American concept of an independent judiciary."

Representatives "have a constituency which numbers in the hundreds to hundreds of thousands, to each of whom they owe fidelity . . . ," they said. "Judges have but one constituency, the blindfolded lady with the scales and sword."

The court also agreed to hear a fifth voting rights case, also from Louisiana, Clark v. Roemer, which concerns a different section of the voting rights law. That case is a statewide challenge to at-large elections for trial and intermediate appellate court judges.