THE average district attorney, said Henry T. Lummus in his classic ''The Trial Judge,'' is like a man with only a revolver facing an angry mob. Lummus, himself a judge, was not trying to glorify prosecutors. His simile referred to the fact - as true today as when Lummus wrote 50 years ago - that if anyone indicted for a crime went to trial, the courts could not possibly keep ahead of the tide.
As a group, therefore, defendants are like Lummus's mob. The district attorney holds them at bay not with force but with his power to decide the chronological order in which cases will come up for trial.
Just as a crowd member hesitates to rush an armed man, out of disinclination to be the lone casualty, defense lawyers do not insist on trial, en masse, because they would rather not face the certainty of giving up a chance at negotiating a sentence. That, among other reasons, is why 85 percent of indictments end in voluntary pleas of guilty.
The right to decide which indictments will be brought on for trial, and the sequence of the trials, is part of a broad range of discretion which district attorneys have traditionally possessed.
Besides the timing of the trials, ''prosecutorial discretion'' covers such issues as whether to bring a complaint to a preliminary inquiry or a probable-cause hearing; whether to seek an indictment from the grand jury (and for what offenses); whether to agree to a reduction of charges in exchange for a guilty plea to a lesser offense; and whether to dismiss indictments voluntarily.
If Massachusetts is any example, American prosecutors have always enjoyed exceptional independence in the administration of criminal justice.
Although judges control the actual trials and the sentences imposed on the guilty, the prosecutor has traditionally answered not to the court but to the public which elected him directly (or which elected the official who appointed the prosecutor).
Besides whatever limitations on prosecutorial discretion may result from public opinion and the prosecutor's desire for reelection, a new development is beginning to restrict the district attorney's freedom and is subtly affecting the judge-prosecutor relationship.
By statutes and rules promulgated with the force of law, the courts - federal as well as state - are being required to limit the interval between indictment and trial. Of course, the Sixth Amendment has always guaranteed every defendant a ''speedy'' trial. Now, however, the Constitution's vagueness has received precise time limits.
The Massachusetts disposition is typical. Excluding certain narrow delays, trial must commence within one year after the defendant has been arraigned (that is, pleaded not guilty). If not, the defendant ''shall be entitled upon motion to a dismissal of the charges.''
This rule has two consequences:
First, no matter how heinous the crime, no matter how strong the government's evidence, if the magic date arrives before trail commences, the defendant walks free.
Second, he walks because a judge must allow his motion; even though the delay lies wholly at the prosecutor's door, the headlines might well read: ''Judge Frees Accused Murderer; Defendant's Motion Allowed.''
Judges, whether life-appointed or (especially) obliged to stand for reelection, dislike being publicly blamed for other people's mistakes. They experience particular discomfort when required to serve as unwilling shields for another public official's miscue.
Thus despite the long tradition of unfettered prosecutorial discretion, trial courts are beginning to assert a more direct control over the pace of criminal prosecutions.
The district attorney can still determine who gets indicted, and for what crime. Thereafter, the speed with which cases move forward and the order in which they come up for trial will rest with the judge.
Henry Lummus's embattled district attorney no longer faces the mob alone.