U.S. District Judge Thomas Penfield Jackson will join prosecutors and defense lawyers today in selecting 12 jurors and six alternates in the case of United States v. Marion S. Barry Jr., a trial that many people in this city had hoped would never be.

Once they are picked, the jurors will be lodged at a hotel and shielded by deputy U.S. marshals from news accounts about the mayor for the entire trial, expected to take at least a month.

With the opening arguments, set for tomorrow, both the defense and prosecution in the case are expected to rush for the high ground in an attempt to describe the other as the true villain.

All along, U.S. Attorney Jay B. Stephens has referred to the mayor's alleged drug use as just another form of corruption of a public official. At the same time, Barry and his supporters have lashed back at Stephens, claiming the presidentially appointed prosecutor would stop at almost nothing to ensnare the mayor with criminal charges.

During the five months since Barry was arrested, captured smoking crack by hidden FBI cameras at the Vista Hotel, the mayor has fought the criminal charges almost as if he were waging a political campaign. Last week, he dramatically took himself out of the running for a fourth term, but his public appearances since then have shown he still commands a strong following.

Yesterday, Barry attended Metropolitan Baptist Church in Northwest with Jesse L. Jackson, and Barry became a member of the congregation. He shook hands, kissed cheeks and offered prayers at the morning service. The scene of well-wishers crowding around Barry resembled many others across the city since his return in March from alcoholism treatment.

In the past, Barry has attended All Souls Unitarian Church on 16th Street NW.

Barry said over the weekend that he was at peace with his decision not to run again, and in a brief interview yesterday he talked of staying "involved with the city and serving people any way I can, anytime I can."

In the meantime, opening statements are set to begin tomorrow in the case, in which Barry is charged with three felony counts of perjury, 10 misdemeanor cocaine possession counts and a count of conspiracy to possess cocaine, also a misdemeanor. Sources familiar with the case said Assistant U.S. Attorneys Judith E. Retchin and Richard W. Roberts are prepared to come out swinging.

Because the indictment purposefully was written vaguely, the opening statement will give prosecutors their first chance to lay out the specifics in their case, interpreting events and using nuance to put the mayor in the worst possible light.

The government lawyers will say there are many reasons to believe Barry was a confirmed, regular user of cocaine. So many close Barry associates, female friends and business acquaintances -- not to mention the FBI cameras -- could not have conspired to lie, the prosecutors will argue.

Barry's lawyers, R. Kenneth Mundy and Robert W. Mance, are prepared to assert just as strongly that it is the government and its chief witnesses who should be on trial. While the prosecutors build their case, leading up to the climactic playing of the FBI tape from the Vista Hotel, Mundy will try to convince the jury it was the government, not Barry, that was out of control.

A master courtroom tactician, Mundy is expected to advance much of his case in cross-examination of the prosecution's witnesses.

Some of Mundy's attacks on witnesses will be frontal assaults on their credibility. In the case of some secondary witnesses, Mundy will depend on their credibility to cast doubt on the stories of the main witnesses, sources said. Mundy may not be able to challenge all of the witnesses, sources said, and may fall back to a position of defending Barry against the most serious of the charges, the three felony perjury counts.

For example, Mundy is expected to devote his lengthiest cross-examinations to three prosecution witnesses: Charles Lewis, Hazel Diane "Rasheeda" Moore and Hassan H. Mohammadi. All three have damaging things to say about Barry, sources said. At the same time, all three could become uncomfortable under Mundy's intense style of questioning.

Of the three, Mohammadi is perhaps the most damaging to Barry, sources said, because he has told authorities he supplied cocaine to Barry regularly during the past several years. The sources said Mohammadi has given exacting details about the mayor's alleged drug use.

But Mundy could damage Mohammadi in the eyes of the jury by pointing out his past. Mohammadi has been a government informer before. In 1982, he pleaded guilty to fraud charges in connection with the bribery of a federal official, and avoided prison by testifying against his accomplices. And last month, in connection with the Barry case, he pleaded guilty to a misdemeanor cocaine conspiracy charge in return for prosecutors' agreeing not to pursue more serious felony charges.

Moore also could be a double-edged sword for the prosecution. She has told authorities intriguing stories about her once-close relationship with Barry, about their drug use and sexual practices, sources said. Prosecutors are expected to laud the born-again Washington native as a courageous woman. But the defense is expected to hammer at her history of drug use and describe her as a pathetic, penniless, strung-out former model ripe for manipulation by the FBI.

Lewis, the former city government employee whose December 1988 brush with D.C. police at the Ramada Inn launched the Barry investigation, could be the pivotal witness. After being convicted of selling cocaine last year, Lewis broke the government's investigation open 10 months ago with his decision to cooperate. To convict Barry of the three perjury charges, the jury will have to believe Lewis.

Although sources said prosecutors can present at least five witnesses to corroborate portions of Lewis's testimony that Barry used drugs with him at the Ramada and in the Virgin Islands in 1986 and 1988, Mundy could have room to maneuver. Sources said the defense will pick at parts of Lewis's story by focusing on the slight differences in the accounts of corroborating witnesses.

Other prosecution witnesses, such as political activist Doris Crenshaw, Washington lawyer Lloyd N. Moore Jr. and Barry friend Theresa Southerland, could be more difficult to cross-examine, sources said. Each has specific allegations of Barry drug use that Mundy and Mance might not be able to contradict, sources said.

Still, some prosecution witnesses, while testifying about the mayor's alleged drug use, also may provide testimony praising Barry, a man who many of his witness friends believe led the city well through a crucial chapter in its history.