LEGISLATION PENDING in Congress to change the rules under which federal grand juries operate got a helping hand Wednesday when the American Bar Association approved a set of principles for grand-jury reform. Chief among them is a recommendation that a witness be allowed to have a lawyer present when being questioned in the grand-jury room. This recommendation, if accepted by Congress - and we hope it will be - should go a long away toward climinating the abuses by prosecutors that have made the grand-jury system suspect. In doing so, it may save the grand jury from extinction.

Originally created in England as a mechanism for protecting citizens against an overbearing government, the grand jury has been used instead by many prosecutors as an instrument for harassing witnesses and browbeating defendants before they are indicted. The Nixon administration was especially skillful at using grand juries for political rather than lawenforcement purposes.

While the Department of Justice has recognized the need for new rules to end such abuses, it is not prepared to go as far as the Bar Association in fundamental reform. Attorney General Griffin Bell, in fact, argued before the lawyers against giving witnesses the right to have a lawyer with them - at present, witnesses' lawyers are required to wait outside the grand-jury room. His fear, and that of many prosecutors, is that grand-jury proceedings will come to resemble those of a trial, thus making the task of law-enforcement officials even more formidable than it is now. We think such fear is exaggerated. But even if it were not, this basic change in the law would be needed to preserve the grand jury's intended function.

Grand juries have been much criticized in the past few decades. They were abolished in England in 1933 and have been eliminated in several states. Their life expectancy in the remaining states and in the federal system was not considered great a few years ago because of the ways in which the juries had been abused. But that has changed. The Watergate affair provided a striking example of their usefulness in investigating and bringing indictments in situations where no other technique could have enjoyed public confidence. They are worth saving - but only if they are restored to the function intended by the Bill of Rights. The best way to do that would be for Congress to pass legislation along the lines suggested this week by the nation's lawyers.