William Simons, head of the Washington Teachers Union for the last 20 years, was defeated for reelection last night by the man who served as his assistant for 14 years.

Preliminary unofficial results gave Harold Fisher, 47, a margin of more than 3 to 2 over Simons, 60. Two other candidates, Walter Cooke and Richard Graham, received a handful of votes among the nearly 2,300 ballots cast.

Fisher received 1,411 votes to 829 votes for Simons, and Fisher's entire slate of candidates for the other eight top union officers won by similar margins, according to uncertified vote tallies.

"I am very happy, of course," Fisher said during a victory celebration at the Dupont Plaza Hotel, where votes were being counted by the American Arbitration Association, which conducted the mail-in balloting for the two-year term.

"I really appreciate the membership's confidence in me and I will certainly strive to make sure that their confidence is well placed."

Simons, who successfully fought to overturn a 1982 union election that gave the union presidency to a different challenger, reacted bitterly last night to news of his defeat.

"The teachers have elected a liar and a thief and may the Lord take a liking to them who elected him," Simons said. He refused to elaborate.

His comment reflected the acrimony that had marked the election since Fisher announced his candidacy in January.

Shortly after Fisher declared his intention to seek the union presidency, he was fired by Simons, who called him "incompetent" and "insubordinate" and ordered him to stay away from union headquarters at 2101 L St. NW.

Union members later voted to order Simons to reinstate Fisher, but Simons refused, saying that the vote was "unconstitutional."

Fisher, who during his campaign for the union presidency had accused Simons of being "too cozy with the administrators and management of the schools" and of being "no longer effective," said after his victory that their differences did not "need to be discussed" further.

"I have not had a chance to analyze the results, but I imagine that the members' ardent desire for a change and a more aggressive leadership" was the key to his success, Fisher said.

"I think they saw in me the opportunity to assert themselves and I certainly hope to live up to that."

He said the most serious challenge facing the teachers union, which has about 4,500 members among the District of Columbia's 5,700 public school teachers, is from the National Education Association.

The NEA, which represented D.C. teachers prior to 1967 when they won the right to bargain collectively, is seeking to regain their representation. About 2,000 teachers have petitioned the Pubic Employees Relations Board, the city's labor practices panel, in an effort to oust the current teachers union, which is affiliated with the American Federation of Teachers.

The board is expected to set a date soon for an election to allow teachers to choose between the two groups.

It was the challenger whose 1982 election victory over Simons was overturned -- James D. Ricks -- who spearheaded the NEA effort. Ricks, a high school teacher who had tried repeatedly to defeat Simons for the union presidency and won a 1,146-to-1,142 initial victory in the 1982 voting, took a leave of absence last spring to become a paid organizer for the NEA.