WASHINGTON — The Supreme Court ruled Thursday that a defense attorney may not arbitrarily exclude potential jurors on the basis of their race.

The justices, by a 7-2 vote, said that lawyers for defendants violate the constitutional rights of would-be jurors and erode public confidence when they make peremptory, or automatic, challenges of prospective jurors based on race.

The ruling bolsters a series of opinions issued over the last six years in which the court has imposed similar restrictions on prosecuting attorneys.

"Be it at the hands of the state or the defense, if a court allows jurors to be excluded because of group bias, it is a willing participant in a scheme that could only undermine the very foundation of our system of justice--our citizens' confidence in it," Justice Harry A. Blackmun wrote for the court.

Blackmun flatly rejected the notion that the accused in a criminal trial should be allowed to shape the jury according to race in an effort to get a more sympathetic jury. "It is an affront to justice to argue that a fair trial includes the right to discriminate against a group of citizens based upon their race," he said.

In the past, defense attorneys generally have been given broad leeway in rejecting potential jurors for any reason. Under Thursday's ruling, a defense attorney would have to be able to show that he had a valid reason, apart from race, for asking that a potential juror be barred from a jury.

Experts said that the decision could make it easier to avoid all-white juries in racially charged cases, such as the retrial of a white Los Angeles police officer charged in the beating of motorist Rodney G. King. But, they said, it could pose a problem in the far more common situation in which minority defendants attempt to exclude some white jurors to ensure that minorities serve on their juries.

The high court decision came in an Albany, Ga., case in which three white defendants were charged with assaulting a black couple. Prosecutors sought to bar the defendants from using their peremptory challenges in an effort to obtain an all-white jury.

The black couple, Myra and Jerry Collins, had gone to a dry cleaning business in January, 1990, to pick up their clothing and allegedly were attacked by the store's owners--Thomas, William and Ella McCollum.