D.C. Mayor Marion Barry's defense lawyer asked a federal judge yesterday to throw out Barry's cocaine possession conviction and schedule a new trial on that count because of "irregularities and contamination of the jurors."

The move could prompt U.S. Attorney Jay B. Stephens to reconsider his decision not to seek a retrial of Barry on 12 charges that deadlocked jury deliberations in August, sources familiar with the case said.

Barry is running for an at-large D.C. council seat on Nov. 6 as an independent, and the maneuver by Barry's lawyer, R. Kenneth Mundy, could delay Barry's sentencing until after the election.

Barry was convicted on one charge of cocaine possession and acquitted on a second. The jurors were unable to reach a verdict on the remaining 12 charges, including three perjury counts. Stephens has said he would not seek a retrial of the charges, but sources said last night he probably would reconsider that position if U.S. District Judge Thomas Penfield Jackson grants Mundy's request for a new trial. Stephens was unavailable for comment last night.

Citing a report by The Washington Post, Mundy stated that a deputy U.S. marshal guarding the sequestered jurors told at least one of them during deliberations that some of the six alternates who had been excused would have convicted Barry.

Additionally, Mundy said "at least several" jurors witnessed police efforts to conduct a sting operation against drug suspect Brian Tribble, a figure in the cocaine death of Maryland basketball star Len Bias in 1986, while the jurors were sequestered at a New Carrollton hotel.

The Post reported Aug. 23 that several jurors were aware during their deliberations that an undercover drug operation had taken place in the parking lot of the hotel where they were staying.

Juror Patricia Chaires said she and juror Valerie Jackson-Warren witnessed the operation from a hotel window and quickly reported it to the marshals out of concern that someone might be trying to sabotage the deliberations. Other jurors said they were informed about the drug sting during breakfast the next morning.

The jurors said they were surprised to learn that U.S. Marshal Herbert M. Rutherford III and Jackson had said the next day that the jurors were unaware of the law enforcement activity.

Authorities said they were unaware of the jurors' presence at the hotel when they planned the Tribble sting there.

"I saw it happen," Chaires said in an interview after the verdict. "I read in the paper afterwards that the judge was told we didn't know about it, but that wasn't true. I didn't know who was being busted, but I knew it was a drug bust . . . .

"We were sitting in a room watching television and I heard a bang. You know, boom . . . . I ran over to the window and looked out and I saw this car being rammed and then take off," Chaires said. "Then I saw a guy on the ground. Valerie saw it too. She said, 'He's being arrested.' Then I saw the jacket with the DEA on it, the FBI, P.G. police, the helicopter starts flying around. I said, 'Oh, my.' I said, 'Valerie, this is a drug bust.'

"I instantly thought . . . this is a set-up . . . . I said I'm sure this has implications. We went to the front desk and the marshals were leaning out the window, they witnessed it."

One juror also said that after the alternate jurors were dismissed at the end of testimony, one marshal told the juror that two of the alternates had gone on television and pronounced the mayor "guilty, guilty, guilty." Other jurors and a courthouse source said marshals kept the judge informed of the status and tone of jury deliberations.

The jurors were sequestered in a hotel and barred from reading or watching any news stories about Barry during the testimony and deliberations in an effort to keep the jurors from considering anything but the courtroom evidence.

Also yesterday, Mundy asked for a postponement of Barry's hearing Friday on objections to the presentence report.

Jackson had ordered probation officials to file the report by yesterday, but the report had not been filed late yesterday and the probation office had asked Jackson for additional time to complete the report.

Jackson had not ruled on the probation officials' request late yesterday.