Federal prosecutors, departing from their original plan, are expected this afternoon to call to the stand Hazel Diane "Rasheeda" Moore, the former girlfriend of D.C. Mayor Marion Barry who worked as an FBI informer in the Jan. 18 sting at the Vista Hotel.

It was clear from the government's opening statement a week ago that prosecutors think Moore, 39, could be their star witness. It is through her testimony that prosecutors plan to introduce the tape of Barry smoking crack in Moore's hotel room, where he went at her invitation. Originally, prosecutors had planned to save Moore for the end of the case, but they changed their plan to maintain the momentum they think they have established during the first week of testimony in Barry's drug and perjury trial.

Some images of Moore are fixed in the city's consciousness: her sexy cover photo in Essence magazine, her stepping down an airplane stairway dressed in a loose pantsuit and wearing sunglasses on her return to California after the arrest. A few details of her background, including the city contracts she obtained through her relationship with the mayor to conduct fashion shows, have been reported.

Virtually everything else about the former model and design consultant remains almost as much of a mystery as on the night of the Vista sting.

It's that mystery, and the details she can provide about her clandestine relationship with Barry, that prosecutors hope can deliver the legal equivalent of a knockout punch, according to sources familiar with the prosecution.

For example, Moore will have to answer these questions that bear on her knowledge and credibility:

How long did she have a romantic relationship with Barry?

Prosecutors have described Moore as having a long-term personal relationship, but it has remained unclear how the two met, came to know each other and developed a romantic bond.

How was she able to conceal the relationship for so long?

With the exception of her immediate family and a few friends, Moore apparently was able to keep this side of her life hidden. For Barry's part, he usually referred to Moore as "R.C.," a nickname that kept even his close friends and associates in the dark. In fact, prosecutors are prepared to show that Barry used nicknames and initials as a way of concealing the identities of his inner circle of friends.

How significant was drug use to her relationship with Barry?

According to sources familiar with the case, Moore has provided exacting details about cocaine binges she took with the mayor in Washington, the Virgin Islands and other locations. Defense lawyers are prepared to fight prosecution efforts to introduce much of these detailed accounts, many of which include graphic descriptions of sexual encounters, on the grounds that the testimony would be unfairly prejudicial to Barry.

How did the relationship end?

Barry's lawyers have said in court that the mayor had not seen Moore during the several months before the sting. According to an FBI transcript of the Vista videotape, Barry at one point tells Moore he still loves her, in spite of the way he treated her. "Once in love, always in love," Barry said.

Indications of the depth and longevity of the Barry-Moore relationship became apparent over the weekend. The Washington Post reported yesterday that, according to logs of Barry telephone messages maintained by the District's Office of Emergency Preparedness, there were at least 214 after-hours and weekend telephone calls to Barry from Moore during the 45-month period from June 1986 to just before the sting operation in January. Moore was the most frequent caller by more than twice as many calls as anyone else listed in the logs.

There could be other questions for Moore as well. Defense lawyers are expected to paint Moore as a down-on-her-luck model who was struggling to escape eviction from her Los Angeles apartment when the FBI knocked on her door in January.

What few details are available about Moore's life in California appear to bear out that image. When the FBI came knocking, Moore was living in an unexceptional, $640-a-month garden apartment in Burbank where she used a cardboard box for a coffee table, according to neighbors. She shared the apartment with her three young children, a man named Donald Bruce Dangerfield, who was listed as a boyfriend on her apartment application, and an unidentified second man who stayed at the apartment sporadically, said Arthur Rossiter, the building manager. Rossiter described Moore's home as "sparse in appearance."

The second man, who Moore referred to as an ex-husband, was not listed on her apartment application, Rossiter said.

Rossiter said he initially rejected Moore's apartment application in mid-December, saying that she listed too many occupants and too little income. Moore, Rossiter said, failed to meet the David N. Schwartz management company's income requirements.

Moore challenged Rossiter's veto and visited the Schwartz company's office in Glendale, where she persuaded employees to rent her an apartment. "She went straight into the office . . . even though she knew we didn't want her here," Rossiter said. Schwartz company employees acknowledged that they overturned Rossiter's veto but declined to comment further.

Neighbors said Moore would leave in the morning carrying a portfolio and wearing her signature hat. She lived in the apartment for two weeks before her arrest for driving under the influence on New Year's Day.

Moore virtually disappeared after the arrest, leaving behind most of her belongings and ceasing her rent payments, prompting the Schwartz company to file for her eviction in February, Rossiter said. He said some "men wearing suits" eventually collected Moore's possessions and paid her back rent in cash in March.

Moore has declined all comment since the Vista sting, short of releasing a statement through her lawyers that said, "While the justice process is underway . . . she does not wish to comment about her personal life."

Moore's family, which claims that she has been unjustly portrayed as a "vixen," also declined to discuss her role in the Barry arrest. Her sister, Rhea, said last week, "As a family, our trust right now is centered completely on the Lord. The criticism and innuendo . . . the talk or stories don't concern us.

Almost certainly, the defense will try to prove that Moore was a paid informer who received help from the government to fight her own minor legal battles.

Additionally, the defense is expected to question Moore about her ability to retain the prestigious firm of Pepper, Hamilton & Scheetz. A. Raymond Randolph, a lawyer in the firm's Washington office, declined to comment yesterday about Moore.