D.C. Mayor Marion Barry yesterday fired Marie Dias, one of his principal aides, after she was quoted as saying that Barry had threatened to dismiss her if she ran next year for the post of nonvoting D.C. delegate to Congress.

Barry reportedly wanted to discourage Dias from running for the congressional seat currently held by Walter E. Fauntroy, a popular District politician whose support Barry has been seeking in what is expected to be his own bid for reelection as mayor. Fauntroy has not yet announced for reelection, but is generally expected to do so.

Herbert Reid, Barry's legal counsel, said yesterday that Dias was not fired because of her plans to challenge Fauntroy, but because of the way she allowed her private dispute with Barry to become public.

"This action by the mayor has nothing to do with politics or political beliefs," Reid said. "Special assistants serve at the pleasure of the mayor. bThere were matters of mutual confidence and reliability that were brought into question."

Dias could not be reached for comment.

She was notified of her dismissal shortly after 6 o'clock last night, Reid said. She will stay on the payroll for 15 more days. As a political appointee, she is not protected by civil service rules against such a dismissal.

As a $47,000 aide with an office inside Barry's District Building executive suite, Dias was Barry's liaison with women, Hispanics, homosexual groups and the elderly, all important segments of the population that could play a vital role in a city-wide election.

Dias, in published reports, had accused Barry of threatening to fire her if she did not drop her planned challenge to Fauntroy. Reid said that "from the mayor's position, some of this [report] was not true."

Barry apparently was upset with Dias for going public with the charge of the purported threat.

"This incident, and the conduct surrounding it, made her usefulness as a special assistant no longer necessary," Reid said.