Federal prosecutors urged a judge in the Virgin Islands yesterday to give Mayor Marion Barry's key accuser, Charles Lewis, "very favorable consideration" in return for Lewis's assistance in the case against Barry.

U.S. District Judge Stanley S. Brotman, a federal judge from New Jersey who is temporarily assigned to the Virgin Islands, has scheduled Lewis's sentencing for today in St. Thomas. Sources said the U.S. attorney here, Jay B. Stephens, had hoped to delay Lewis's sentencing until after Sept. 17, when he has said he will announce his decision about whether to seek a retrial in the case against the mayor. Barry was convicted of a single count of cocaine possession and acquitted of a second by a jury that was unable to reach a verdict on the remaining 12 charges that Barry faced in his two-month trial that ended Aug. 10.

Lewis was convicted in the Virgin Islands in April 1989 on cocaine possession and distribution charges, but his sentencing there was delayed pending the outcome of the case against Barry. Lewis pleaded guilty here to additional drug charges as part of an agreement with prosecutors to cooperate in their investigation of Barry.

Under federal sentencing guidelines, Lewis could be sentenced to more than four years in prison, but Stephens promised to seek leniency for Lewis in return for his cooperation. That promise could mean that Lewis, who in June completed a 13-month sentence for his drug conviction here, could receive credit for his time already served and would not face a return to jail.

In a memorandum, filed by the U.S. attorney for the Virgin Islands, Terry M. Halpern, the prosecutors said Lewis "testified truthfully {at Barry's trial} no matter who questioned him, whether it was the government, the defense or the court. The prosecutors and the investigators who worked on the case believe that the defendant cooperated fully and completely with the government."

The prosecutors said Lewis's "assistance was invaluable in the prosecution of a public official who has been violating narcotic laws and making false declarations under oath to conceal his drug abuse.

"Additionally, the defendant's cooperation caused other witnesses to cooperate and provide truthful information. Moreover, the defendant genuinely appears to be remorseful and has accepted full responsibility for his criminal conduct."

In addition to providing evidence about Barry's drug use, Lewis gave prosecutors information about abuses in the D.C.-Virgin Islands personnel project of which Lewis had been a co-director, the prosecutors said. Barry was not implicated in the federal investigation of that project, and no charges have been filed in connection with it. The District government has said it is owed almost $200,000 from the Virgin Islands government, reimbursements that Virgin Islands officials have said they will not make.

The federal sentencing guidelines allow a federal judge to depart from the sentencing range if a prosecutor makes a written request and states that the defendant has provided significant assistance to a government investigation. In the cooperation agreement, Stephens agreed to make that request, but had delayed doing so until the Barry case was completed.

For Stephens, Brotman's decision to sentence Lewis today posed a dilemma, sources said. On one hand, Stephens would prefer that the sentencing be delayed until after he announces his decision about a possible retrial because prosecutors have been able to maintain leverage over Lewis through the cooperation agreement.

If they seek leniency for Lewis, it is likely that Lewis will not be sentenced to prison. Prosecutors worry that a free Lewis could be a less forceful witness against Barry, sources said.

On the other hand, sources said, if the prosecutors had not filed the request for leniency, the judge would have little choice under the guidelines but to send Lewis to prison. If that were to happen, the sources said, the credibility of Stephens and the U.S. Attorney's Office among prospective witnesses could suffer significantly.

Barry's lawyer, R. Kenneth Mundy, spent much of his cross-examination of Lewis asking about the agreement with prosecutors. Several times, Mundy waved the agreement in the air and pointed at Assistant U.S. Attorneys Richard Roberts and Judith E. Retchin, noting they were waiting to see how Lewis testified before making the request for leniency.

Neither Mundy nor his partner, Robert W. Mance, was available for comment yesterday.