Of the 14 criminal charges facing D.C. Mayor Marion Barry, the first count in his indictment -- conspiracy to possess cocaine -- may be the least of them. A conspiracy, for all its sinister implications, is no worse a crime than its object, and possession of cocaine is a misdemeanor.

But the conspiracy charge in Barry's case has loomed far larger than the one-year maximum prison sentence that could come with it. Prosecutors have used it throughout the trial to put damaging evidence before the jury that could never be admitted in an ordinary drug and perjury trial.

Yesterday's installment was a videotape of the mayor's public denials from June 1987 through September 1989 of drug use. In one of them, Barry says that "never in my life" has he used illegal drugs.

Coming as it did after voluminous evidence of Barry's alleged drug use, the videotape was what lawyers call "prejudicial": By making the mayor look bad, the tape may influence the jury against him. At two bench conferences and an exceptionally heated argument with the jury out of the room, defense lawyer R. Kenneth Mundy argued unsuccessfully that the prejudice of the tape outweighed any value as evidence.

"The whole purpose for the government using that, or seeking to use it, is to hold him up to ridicule and scorn for his hypocrisy," Mundy argued.

Assistant U.S. Attorney Richard W. Roberts did not explicitly deny that his tactical purpose was just as Mundy had described. It is improbable that he could have done so with a straight face.

Nor did Roberts say, as he might have, that turnabout is fair play in a court of law. Much of Mundy's defense to date has been based on emotional appeal.

Appeals to jury sentiment have an oddly ambiguous place in a criminal trial: officially forbidden, yet pervasive. Among the most important skills for a successful litigator is to find legitimate legal reasons to put that kind of evidence before a jury.

For yesterday's videotape display -- as for so much other damaging evidence -- Roberts found his basis in the conspiracy charge. A conspiracy cannot be charged without specifying the "overt acts" used to pursue it, and Roberts cited Barry's public denials of drug use as "one of the means to effect the conspiracy."

The fact that he denied drug use, in other words, was part of the proof of the conspiracy. U.S. District Judge Thomas Penfield Jackson agreed.

Because the conspiracy alleged extended from the fall of 1984 to 1990, it provides a ready framework for any evidence of drug use in that time. Among the examples before the jury:

Hazel Diane "Rasheeda" Moore testified that she had used cocaine with the mayor more than 100 times. Only one of those alleged occasions -- at the Vista Hotel on Jan. 18 -- has been charged in the indictment.

Georgetown restaurateur Hassan H. Mohammadi testified Monday and Tuesday that Barry smoked opium with him on numerous occasions. Barry is not charged with smoking opium. Prosecutors persuaded Jackson to permit the testimony in part because they said it fell within the alleged conspiracy to possess controlled substances.

Washington lawyer Lloyd N. Moore Jr. testified yesterday that A. Jeffrey Mitchell said Barry was hospitalized in Los Angeles after the Superbowl in January 1987 after smoking "cocaine laced with something." Not only is Barry not charged in that episode, but Moore's testimony would ordinarily be inadmissible hearsay. Because Mitchell has been named as an unindicted co-conspirator, however, Jackson overruled the defense objection.

For each piece of evidence, prosecutors could point to a lawful purpose. But in every case the defense had another view.

"It's just smear stuff," Barry co-counsel Robert W. Mance said. "I think what they're trying to do is throw so much stuff out there, they just hope some of it will stick. That's what they do in conspiracy cases."

Staff writer Michael York contributed to this report.