In what is shaping up as a court showdown with far-reaching implications, the Government Accountability Project, a nonprofit watchdog organization, has refused to finger informants who gave it embarrassing evidence about alleged incompetence and safety problems at the South Texas nuclear power plant near Houston. This spring, the Nuclear Regulatory Commission, which is responsible for ensuring the safe operation of the nation's nuclear power plants, slapped GAP with a subpoena demanding disclosure of its sources.

GAP refused to comply with the subpoena. Then the Justice Department entered the case on NRC's side, asking a federal district judge in Washington to order GAP attorney Billie Garde to turn over the information on the South Texas whistleblowers.

The legal action has aroused concern among other public-interest groups. If GAP can be forced to divulge its sources, conscientious employees in government and industry could be discouraged from taking their complaints to outside organizations. Whistleblowers rely on the promise of confidentiality that these watchdog groups provide; it's the only way they can protect themselves from retaliation by the bosses whose incompetence they have exposed.

GAP's investigation of the South Texas situation was no haphazard affair. It involved 36 informants at the nuclear plant, and Garde spent months assembling evidence of the problems there.

Turning the public-interest group's thoroughness against GAP, the NRC and the Justice Department argued in court that the problems GAP found out about could affect the health and safety of the public. Therefore, the argument goes, GAP is keeping the government from correcting the problems by withholding information from the NRC.

In its response last week, GAP declined to swallow the line being offered by Justice Department attorneys. For one thing, the public-interest group pointed out, the NRC already knows the identity of about a dozen employees who have had contact with GAP -- yet commission officials have never questioned them to determine what they may have told the watchdog group. And the information, GAP contends, is protected by an attorney-client privilege between it and the whistleblowers.

As a compromise, GAP has offered to turn over the results of its investigation to an independent investigative entity -- but not to the NRC, and especially not to officials at the agency's Region 4 office in Arlington, Texas. That office not only is in charge of the South Texas plant but has a track record of covering up problems and retaliating against whistleblowers.

Several workers who brought complaints to Region 4 officials in supposed confidence later lost their jobs when their names were leaked to their bosses.

One employee, Charles Atchison, lost his job at the Comanche Peak nuclear plant, near Dallas, after he told NRC officials about defective welds. An internal investigation established that Region 4 officials had indeed broken their promise of confidentiality to Atchison on three separate occasions.

Other employees who helped NRC investigator George Mulley put together massive documentation of lax enforcement in Region 4 were identified when his report was released to Region 4 officials who were targets of the investigation -- without his knowledge. They protested vehemently to Mulley.

''Most of these individuals felt that the distribution was done purposely to expose them to possible future retaliation,'' Mulley testified to a Government Affairs subcommittee chaired by Sen. John Glenn.

The NRC's record on whistleblowers isn't bad just at the Region 4 office. Our associate Stewart Harris recently reported that agency brass in Washington struck back Soviet-style at three employees who testified on NRC problems before Glenn's subcommittee. One witness was ordered to consult a psychologist; the two others had their testimony referred to the Justice Department for possible prosecution on charges of perjury. Both ideas were abandoned.

In the papers filed last week, GAP cites a Supreme Court decision in 1957 that struck down an Alabama court ruling that the National Association for the Advancement of Colored People had to provide a list of its membership to the state. The Supreme Court held that the state's demand impinged on NAACP members' constitutionally guaranteed right to freedom of association. GAP hopes this precedent will protect the confidentiality of its whistleblowers.