The Supreme Court, in a decision likely to enhance prosecutions of those charged with child abuse, ruled 6 to 3 yesterday that defendants in such cases do not have a constitutional right to be present at hearings held to determine a child's competency to testify.

The court, in a decision written by Justice Harry A. Blackmun, rejected arguments by a Kentucky man who said his Sixth Amendment right to confront the witnesses against him was violated when he was barred from attending a competency hearing for two sisters, 7 and 8 years old.

Sergio Stincer, sentenced to 20 years for sodomy, "had the opportunity for full and effective cross-examination of the two witnesses during trial," Blackmun said, overturning a decision by the Kentucky Supreme Court.

The court, which is expected to finish the current term within two weeks, also ruled that:Labor unions may be held liable for claims of racial discrimination if they fail to press discrimination claims against employers. The ruling upheld lower court decisions in Pennsylvania that the United Steelworkers of America and two local unions discriminated against black members when the unions, knowing the employer was discriminating in violation of the contract, failed to act. Businesses engaged in a "closely regulated industry" have only an "attenuated expectation" of privacy in commercial property. As a result, the court said police in New York did not need a warrant before entering a junkyard to search for stolen cars. A rule requiring attorneys who want to join the federal bar in New Orleans to live or have an office in Louisiana arbitrarily discriminates against out-of-state attorneys. The decision is likely to affect similar rules in Virginia, where out-of-state attorneys are barred from the federal courts because of similar requirements. The 6-to-3 ruling in Frazier v. Heebe is likely to spur challenges to the requirement that attorneys be members of the state bar to be admitted to the federal bar.

In the child abuse case, Kentucky v. Stincer, the Kentucky Supreme Court had thrown out Stincer's conviction for sodomizing three young children, finding that his right to confront the witnesses was violated when he was excluded from the pretrial hearing where a judge ruled two of the children competent to testify.

Prosecutors said the ruling would increase trauma and intimidation and chill prosecutions.

Blackmun, in overturing the state court, said "a defendant is guaranteed the right to be present at any stage of the criminal proceeding that is critical to its outcome if his presence would contribute to the fairness of the procedure."

But Blackmun said the questions asked the girls at the hearing had nothing to do with the case but "were directed solely to each child's ability to recollect and narrate facts, to her ability to distinguish between truth and falsehood."

Blackmun said that, based on the facts in the case, "there is no indication {Stincer} could have done anything had he been at the hearing nor would he have gained anything by attending."

Justice Thurgood Marshall, joined in dissent by Justices William J. Brennan Jr. and John Paul Stevens, said Stincer's exclusion "constitutes an intolerable subversion" of the Sixth Amendment's ability to promote a perception that the criminal justice system is fair.

Marshall said the amendment "plainly envisions that witnesses against the accused shall, as a rule, testify in his presence."

In the steelworkers case, Goodman v. Lukens Steel Co., Justice Byron R. White, joined by five justices, said the unions could be held liable for discrimination even though there was "no suggestion . . . that the unions held any racial animus against or denigrated blacks generally. Rather, it was held that a collective bargaining agent could not, without violating {civil rights laws}, follow a policy of refusing to file . . . racial discrimination claims however strong they might be and however sure the agent was that the employer was discriminating against blacks."

A union that fails to assert discrimination claims, White said, either to avoid antagonizing the employer or in deference to its white membership, may be sued under federal civil rights laws, even if the union's leaders bear no animus toward minorities. White was joined in this part of the opinion by Chief Justice William H. Rehnquist and Justices John Paul Stevens, Brennan, Marshall and Blackmun.

The decision in the New York junkyard case, New York v. Burger, involved a law similar to those of several states, including Virginia, to combat car theft. Blackmun, writing for the court, said searches made under the New York law are constitutional because, even if they are made by police rather than by regulators, they fall within the court's previously granted exceptions to the warrant requirement for administrative inspections of "closely regulated" businesses.

In dissent, Brennan, joined by Marshall and, for the most part, Justice Sandra Day O'Connor, said "the implications of the court's opinion, if realized, would virtually eliminate Fourth Amendment protections of commercial enterprises in the context of administrative searches."