Weeks before today's scheduled start of the drug conspiracy trial of D.C. Mayor Marion Barry, U.S. District Judge Thomas Penfield Jackson, who presides over the case, told attorneys that he had a special worry: finding 12 people who have not already made up their minds about Barry's guilt or innocence.

A number of criminal defense lawyers and former prosecutors said that they believe jury selection would be the most important stage in a trial of Barry -- even more crucial than any showing of the FBI videotape of Barry's arrest at the Vista Hotel.

The concern about jury selection stems from the intense emotions the Barry case has stirred in this city. Many black people have said they believe the white establishment is trying to drive the mayor out of office. Many other people, black and white, have said he is a bad role model for children and is bringing disrepute to the city.

Of course, prosecution and defense each hopes to wind up with jurors it believes will lean toward its side. At every criminal trial, each side can excuse potential jurors from a randomly selected pool for "cause" -- a definable reason why they may be expected to have some bias, such as having recently been the victim of a crime. Each side also gets a number of peremptory strikes -- usually 12 for the defense and six for the prosecution -- for which it needs give no reason.

Local lawyers interviewed about the case stated conflicting opinions about what kind of jurors each side should seek in the Barry case. Their answers reflected courthouse folklore and individual styles.

"Jury selection is like phrenology," said lawyer John Mercer. "It's not scientific."

Most of the lawyers interviewed believed that Barry has strong support among many poor and working-class blacks who admire his racial pride and up-from-the-streets, anti-establishment style. These lawyers thought Barry's attorney, R. Kenneth Mundy, should be seeking jurors of that type.

Lawyer Ed Sussman said the ideal jurors for the defense are "people who have been left behind . . . {who} see this in a sense as, 'If you make it, the establishment comes after you.' "

In an interview with The Washington Post on Tuesday, Barry seemed to send a subtle appeal to his core constituency to stand by him. "I think the prosecutors know," he said, "that in this town all it takes {for mistrial} is one juror saying, 'I'm not going to convict Marion Barry. I don't care what you say.' "

D.C. Council Chairman David A. Clarke later said Barry appeared to try to send a message "to those 12 people out there, whoever they are," that they should not convict him, but must acquit him "because of who he is."

Many lawyers interviewed said they think prosecutors would seek more well-to-do, middle-class blacks who are scandalized by Barry's freewheeling lifestyle.

But lawyer Willie Leftwich thinks the ideal prosecution jury is "a bunch of white women from Cleveland Park, the kind that went to Smith or Vassar and are home during the day, and whose husbands are lawyers or accountants. If I were the defense counsel," he said, "I'd be looking for everybody but them."

Leftwich added that he doesn't think Barry should seek an all-black jury. "I would not at all" try to keep whites off the jury, Leftwich said. "Even in the white community, I have detected sympathy for his plight." And, he added, "there are any number of white people in the District who use cocaine."

But the great majority of lawyers interviewed said Barry's best chance for acquittal lies with an all-black jury.

"Obviously, it's not in Barry's interest to have white jurors on the panel," said lawyer Edwin C. Brown. Sussman agreed. "I can't imagine too much dispute over that," he said, adding that whites "have had enough of Marion Barry."

Some lawyers said that attorneys for black defendants in Washington frequently use their peremptory strikes to eliminate whites. A 1986 Supreme Court ruling bars prosecutors from using that tactic, but no such rule bars defense counsel.

Jury selection is expected to be grueling, and to last at least a week. The 12 jurors and two to four alternates will be chosen from a pool of 250 potential jurors. They already have been randomly selected by computer from a list of 80,000 names drawn from District voting and driver's license records.

Prospective jurors will be grilled about what conclusions they may have formed from news reports on Barry, and about their attitudes on issues such as drugs and alcoholism. The questioning, or "voir dire," is expected to start Monday with a written questionnaire and to continue with questioning -- one member of the jury pool at a time -- in open court. "There are going to be some very sensitive questions on that questionnaire," Jackson said last week.

Lawyers were split on the role the sex and age of potential jurors might play. Some lawyers who think Barry has a strong base of support among senior citizens said Mundy may go for older jurors.

How women will view the mayor -- and his betrayal by former girlfriend Hazel Diane "Rasheeda" Moore -- is another matter. Will women be angry at Barry, who is married, for flirting with Moore? Or at Moore, for cooperating with the FBI, a frequent opponent of the civil rights movement?

"That's a ringer," Leftwich said. "My wife says {women} don't see how Effi {Barry} stood beside" him.

Mundy will not be depending on instinct alone. He has confirmed retaining a consulting firm to help in jury selection, but would not name it. Such firms use psychologists to conduct opinion surveys or formulate questionnaires that help lawyers know what different types of people -- reflecting various income levels, church affiliations or political beliefs -- think about a client or a trial tactic. Other consultants talk to potential jurors' friends and neighbors, or drive past their houses to read bumper stickers.

Federal judges have broad authority in setting rules for jury selection, including how deeply each side can delve into sensitive questions such as a potential juror's feelings about race or politics.

Jackson may allow the lawyers to ask about both topics, but only if he finds the questions are germane to the case and are not used to exclude an entire class of jurors -- for instance, all Republicans.

The questioning likely would reveal that many potential jurors are extraordinarily concerned about drugs, and know people who have been involved somehow with drugs, lawyers interviewed said.

"I can't believe the penetration of drugs into this community," Sussman said. Most jurors he questions have a relative affected by the drug trade, he said, "and I'm talking about serious consequences. Prison. Death . . . . But you also have a fairly sophisticated drug jury."