CAN THE government prosecute someone for drug possession when it uses an attractive woman to lure him to a hotel room, loosens him up with a couple of drinks and provides the drugs and the means to use them that form the basis of the charge? Is it fair to target a controversial political figure for an elaborate sting operation in an attempt to obtain evidence of a misdemeanor -- when such an operation has rarely, if ever, been employed for such a purpose against anyone else? Is it unconstitutionally vindictive for the government to levy additional drug charges against a defendant shortly before trial?

The case of D.C. Mayor Marion Barry, now being tried in federal district court, raises these and many other questions about the limits of prosecutorial discretion. Four issues in particular -- entrapment, due process, selective prosecution and prosecutorial vindictiveness -- are being debated from cocktail parties to the judicial bench. Whatever the outcome of the Barry case, it provides a striking example of the extraordinary power society has entrusted to prosecutors. Traps and Pitfalls

The word "entrapment" may conjure up simple images of mice, cheese and the crack of the lever. But as a defense strategy, entrapment is actually far more complex and difficult to establish. It is not enough to show that the government provided the opportunity, or even that it deliberately lured the defendant into committing a crime. Setting a trap is not automatically illegal, even, for example, if the government engages an attractive woman to set the mood. The key to the defense of entrapment is that the accused was not otherwise willing, was not predisposed, to commit the crime.

Using that defense, however, changes the whole complexion of a criminal trail, introducing new elements of ambiguity and risk.

Suppose a defendant is on trial for possessing cocaine at a particular time and place. Federal rules of evidence require that the government cannot try to prove that charge by showing that the defendant has possessed cocaine at other times and at other places. But evidence of prior similar conduct is admissible to rebut a claim of entrapment. Since the government bears the burden of proving that the defendant was predisposed to commit the crime, it can attempt to carry that burden through proof that he has done the same thing before. This can be damaging: Although juries are instructed by the judge that the defendant is presumed to be innocent, in practical terms evidence of prior similar criminal activity is often tremendously prejudicial.

To avoid that problem, defense lawyers sometimes artfully try to suggest an impression of entrapment to the jury without formally raising it as a defense. In Barry's case, the prosecution may have effectively pre-empted such a strategy by returning additional drug possession and conspiracy charges. Now, evidence of drug use prior to the Vista sting will be admissible at trial irrespective of any entrapment defense -- a strategy which R. Kenneth Mundy, Barry's lawyer, recently said he will not pursue. Nonetheless, entrapment haunts the trial.

The defense of entrapment was first recognized and sanctioned by the Supreme Court in a 1932 case growing out of the prosecution of one C.V. Sorrells of Canton, N.C., for possession and sale of a half-gallon of bootleg whiskey. A government prohibition agent, posing as a tourist, visited Sorrells's home accompanied by three of Sorrells's friends. The agent learned that Sorrells was a veteran of World War I and a former member of the same division in which the agent had served, information he later used to ingratiate himself. The agent asked Sorrells if he could get some liquor, but Sorrells said he didn't have any. A second request met with the same result. The conversation then turned to war reminiscences, after which the agent asked Sorrells a third time for the whiskey. This time, Sorrells left and returned a few minutes later with a half-gallon jug, which he sold to the agent for $5. For this grand sum, legal history was made.

The prosecution produced no evidence that Sorrells had ever possessed or sold any liquor prior to the act for which he was charged. It relied on several witnesses who testified that Sorrells had a general reputation as a rum runner. Sorrells showed he had been a reliable employee at the Champion Fibre Company for six years.

In this landmark ruling, the Supreme Court castigated the government for luring an otherwise innocent man into committing a crime by repeated and persistent solicitation, taking advantage of the sentiment aroused by war reminiscences. The court ruled that it is impermissible to use "decoys" to ensnare the innocent, and sent the case back for a new trial so that the issue of entrapment could be determined by the jury -- specifically stating that "the predisposition and criminal design of the defendant are relevant."

The issue of government conduct and the defendant's predisposition would remain controversial {see box} for decades. Forty years after Sorrells, it divided the Burger Court; but the law began to evolve. In a 1973 case, a federal undercover narcotics agent infiltrated an illicit methamphetamine laboratory, and supplied the defendant with an essential, difficult to obtain, ingredient in the manufacturing process. Justice William Rehnquist's opinion for the court rejected the claim of entrapment, reaffirmed the focus on the defendant's predisposition and reinstated his conviction. Four justices dissented on the ground that predisposition was irrelevant, and that the government's participation in the manufacturing process required dismissal of the prosecution.

Rehnquist's opinion, however, raised an interesting new suggestion: that when the conduct of law enforcement agents is "outrageous," the Constitution's Fifth Amendment guarantee of due process of law might bar a prosecution even if the defendant were predisposed to commit the offense. Justice Lewis Powell gave further support to the idea in an influential concurring opinion he authored in a 1976 drug case. Powell said, however, that cases in which such a due-process defense could prevail would be rare. Police over-involvement in the crime would, he wrote, "have to reach a demonstrable level of outrageousness before it could bar conviction." That would be particularly difficult to show with respect to narcotics offenses, which are hard to detect without undercover agents.

Defendants who have raised the due-process defense have learned just how difficult it is to meet that standard. Several Abscam defendants, including former senator Harrison Williams, claimed that they had been unfairly targeted, tempted and "coached" by undercover operative Mel Weinberg before going on video camera. The convictions were all sustained, based on the judgment that the government's conduct had not reached the level of coercion needed to sustain the due process defense. So far, Mayor Barry's due-process claim has been equally unsuccessful. Barry argued that the government was "outrageous" in allowing him to smoke crack cocaine at the Vista. Judge Thomas Penfield Jackson rejected the contention, stating that dismissal of the case would be warranted only if the government's conduct included coercion, violence or brutality to the defendant, something he said Barry has not claimed.

The evolution of these concepts has provided at least a partial resolution of the historic tension in the law. While the traditional entrapment defense still focuses on predisposition to commit the crime (an issue reserved for the jury), courts are now at least willing to listen to the claim that outrageous governmental conduct warrants dismissal irrespective of predisposition. The due process defense, however, is presented only to the trial judge. Prosecution and Prejudice

Shortly after Barry's arrest last January, NAACP Executive Director Benjamin L. Hooks and others speculated that the government's pursuit of Barry might have been part of a larger federal effort to harrass black elected officials through "selective enforcement of the law." Attorney General Dick Thornburgh called the suggestion "sadly misinformed."

Selective prosecution is based on a defendant's claim that he has been specially targeted or singled out by the government. Under the Constitution, every individual is guaranteed the equal protection of the law. But that does not mean that everyone must be treated exactly alike. In order to establish selective prosecution, the defendant has to show three things: that he was in fact treated differently from other similarly situated individuals; that the decision to treat him differently was deliberate; and, most importantly, that the decision to prosecute was based on an improper factor.

In one celebrated case in the late '60s, Jeffrey Falk, a highly vocal draft resister, was prosecuted and convicted in Chicago federal court for failure to possess a draft card. Falk appealed, claiming selective prosecution. The Court of Appeals reversed the conviction because Falk had not been allowed to try to show that the government had adopted a policy of not prosecuting most draft-card cases, yet had deliberately singled him out. In its decision, the court stated that an attempt to suppress or intimidate First Amendment activities would warrant dismissal of the prosecution.

In Barry's case, the issue is not simply selective prosecution: Many people are prosecuted for unlawful cocaine possession. Rather, the question would be whether there was improper selective use of a very expensive and elaborate sting operation in order to catch somebody in possession of a small amount of cocaine. But a decision to investigate is not necessarily the same as a decision to prosecute. Even if Barry could show that he was unjustifiably singled out, he would have to persuade the courts to extend the selective prosecution principle to selective investigation.

A related concept involves vindictive prosecution. The mayor has argued that the government acted unfairly in adding new charges shortly before his scheduled trial on pending charges. In certain circumstances, defendants have been successful in moving for dismissal of new charges on the ground that the prosecutor who brought them acted vindictively. The due-process clause of the Constitution prohibits a prosecutor from retaliating against a criminal defendant for exercising constitutional or other rights. For example, if a defendant is convicted but subsequently gets the charges dismissed on appeal, the prosecutor cannot then indict the defendant on new, more serious charges, without showing that they are based on evidence discovered after the trial.

Billy Wayne Miracle was indicted in Potter County, Texas, in 1974 on a charge of second-degree robbery, which carried a possible sentence of 99 years. A second indictment arising from the same incident was returned against Miracle before trial, charging aggravated robbery, with a mandatory life sentence. The prosecutor chose to proceed on the first indictment, and the jury found Miracle guilty. Miracle succeeded in having the conviction vacated because of improper jury deliberations. At the second trial, however, the state elected to prosecute Miracle on the more severe second indictment; he was convicted again and received the automatic life sentence. A federal appeals court vacated this second conviction on the ground that the prosecutor had unconstitutionally "upped the ante" against Miracle after he had succeeded in vacating the first conviction.

Mayor Barry moved to dismiss the second set of charges against him on grounds of prosecutorial vindictiveness. Judge Jackson denied the motion, saying that Barry had known all along that there was an ongoing grand jury investigation that could result in additional charges.

In most cases, when new charges are added as a defendant approaches trial, the defendant can protect himself only by moving to delay trial on all charges, or, as Barry attempted unsuccessfully, to seek separate trials of the old and new charges. Trial judges are generally given wide latitude by appellate courts in deciding these kinds of scheduling issues. Denial of a defendant's motion for delay or severance rarely leads to reversal on appeal, unless he can show serious harm, such as that the refusal to delay has interfered with his ability to retain counsel.

No matter how it is resolved, the Barry trial will be over in a few weeks. But the questions it raises -- especially the almost boundless authority and discretion American society has given prosecutors -- will be with us for years to come.

Disorder in the Court

THE COURT'S focus on predisposition in the 1932 entrapment decision was highly controversial at the time, and it remained so for decades. Justice Owen W. Roberts, in a separate concurring opinion, agreed that Sorrells's conviction had to be reversed, but vigorously disagreed with the majority's reasoning. Roberts, who was joined by Justices Louis D. Brandeis and Harlan F. Stone, insisted that Sorrells's predisposition to sell liquor was beside the point. In his view, the courts were obligated to protect the purity of governmental processes from reprehensive methods irrespective of the defendant's reputation or prior record. In other words, Roberts said, if the government's conduct was bad enough, entrapment would be an absolute defense.

Furthermore, he argued that the question of governmental conduct was a matter to be decided by the trial judge, not by a jury. He wrote, "it is the province of the court and the court alone to protect itself and the government from such prostitution of the criminal law."

Since then, the Supreme Court has remained divided on the issue. In a 1958 decision, narcotics addict Joseph Sherman and a government informant met at a doctor's office where both were seeking rehabilitation. The informant repeatedly asked if Sherman had a good source for narcotics; Sherman tried to avoid the issue. The informant asked again, complaining of his suffering. Sherman gave in, and did narcotics with the informer. The informer then tipped federal narcotics agents, who observed Sherman giving narcotics to the informer in exchange for funds supplied by the government. Sherman was convicted despite his claim that he had been entrapped.

In an opinion by Chief Justice Earl Warren, the court unanimously reversed the conviction ruling there wasn't enough evidence of Sherman's predisposition to satisfy the government's burden of proof. All the government had shown was that Sherman had been convicted of selling narcotics nine years previously and possessing them five years previously. The court was particularly critical of the government's enticing someone into returning to a habit he was attempting to overcome.

Four justices, led by Justice Felix Frankfurter, again stressed the issue of impermissible government action. "No matter what the defendant's past record and present inclinations to criminality," Frankfurter wrote, "or the depths to which he has sunk in the estimation of society, certain police conduct to ensnare him into further crime is not to be tolerated by an advanced society." He also insisted that the question of whether the government's conduct was intolerable should be decided by the trial judge, not the jury.

Peter Isakoff is a partner in the Washington law firm of Ben-Veniste & Shernoff.