As the trial of Oliver North continues, the threat that Attorney General Richard Thornburgh will abort the proceedings on national security grounds hangs over the courtroom.

In weighing the potential danger of releasing classified information against the sanctity of our legal system and the public's right to know, he would do well to review the 1976 Richard Helms case, when the Justice Department, over the intelligence community's objections, actively attempted to prosecute crimes committed during a covert action.Thornburgh was then the assistant attorney general pursuing the prosecution of Helms, the former director of central intelligence, as well as International Telephone & Telegraph officials charged with lying to Congress about efforts to overthrow the Allende government in Chile.

In May 1976, Thornburgh asked the Central Intelligence Agency to make its agents available to Justice Department lawyers for questioning and to declassify more than 500 documents to be used for indictments and trial. The intelligence community, represented by the director of central intelligence, George Bush, dragged its feet.

Bush insisted that as head of the intelligence community he must have final say on what classified information could be used by the Justice Department.

Attorney General Edward H. Levi responded that President Gerald R. Ford "had directed that national security considerations would not be used to prevent the investigation and prosecution of illegal activities."

In November 1976 - after six months of wrangling with Thornburgh - Bush reluctantly began turning over excised materials. Nevertheless, the Justice Department accused Bush's office of stalling, of deleting documents in a "casual" and "cavalier" manner and of withholding information.

Thornburgh took his concerns to the White House counsel, Philip Buchen, and requested a direct presidential order to Bush.

In the end, Ford approved the release of the information and Thornburgh pursued the prosecution, using most if not all of the material Bush had continued to claim was sensitive.

By then, however, Jimmy Carter had defeated Ford. Helms cut a deal with the Carter administration and pleaded nolo contendere (no contest) to two charges of not testifying "fully and completely" to Congress, received a $2,000 fine and announced he would wear his conviction "like a badge of honor."

ITT officials successfully resorted to "gray mail" - the threat of exposing supposed national security secrets at a trial - and all charges were dropped or dismissed.

Oliver North's attorneys have also effectively used "gray mail." They intend to show that he mimicked his superiors - including the former and current presidents - in keeping from Congress the fact that the Reagan administration was secretly providing money, arms and CIA services to third countries as bribes in return for assistance to the Contras.

Once disclosed in general terms, however, these embarrassments (which may or may not have been impeachable offenses) do not provide the attorney general with national security grounds for avoiding their use at trial.

Thornburgh, Levi, Buchen and Ford all accepted a single line of reasoning in 1976: The public has a right to know what its government does in its name and a right to a judicial system that holds officials accountable.