William Simons, president of the Washington Teachers Union for 20 years who was defeated in a stunning election upset Thursday night, said he would immediately relinquish his duties to the winner, Harold Fisher.

Simons' lengthy tenure in the presidency and a belief by many teachers that Fisher would do a better job of unifying the 4,500-member union to defeat a bid by the National Education Association to become the teachers' bargaining agent were the major reasons for his defeat, according to several teachers and union observers.

Simons, 60, the only president the union has had in its 20-year history, was defeated by a 1,411-to-829 vote, according to a spokeswoman for the American Arbitration Association, which tallied the mail-in ballots at the Dupont Plaza Hotel late Thursday night.

Yesterday, Simons refused to say whether he would challenge the election results, but said he would immediately turn over the decision-making to Fisher and would support Fisher's efforts to defeat the NEA.

Simons successfully fought to overturn a union election in 1982 that gave the presidency to challenger James D. Ricks.

"I believe in the union," Simons said. "I didn't spend my life building this union to see it fall by the wayside. I appreciated the opportunity to have served the teachers of D.C. for 20 years. Fisher takes office on July 1, but I have finished making decisions in order to bar off any confusion.

"C'est la vie . . . It the defeat had to come sometime. I've been preparing myself . . . . "

Simons, 60, became synonymous with the teachers' union after winning its fight for collective bargaining in 1965. In the years that followed he aggressively led the union through several teacher strikes and negotiated contracts that made District teachers among the highest paid public school instructors in the country.

The contest between Simons and the man who served as his assistant for 14 years was marked by acrimony and discord.

Last February, shortly after Fisher, 47, announced his candidacy for the union presidency, he was fired by Simons, who called him "incompetent" and ordered him to stay away from the union headquarters at 2101 L St. NW.

About 100 union members voted to order Simons to cease all disciplinary actions against Fisher and rehire him. Simons refused.

Simons' actions were "a clear attempt . . . to discredit my campaign," Fisher said.

Fisher credited his victory to his reputation as an aggressive field representative "who is dedicated to teachers" and the widespread dissemination of his campaign literature to schools and homes of teachers. Observers said a solid and committed network of teachers who campaigned for Fisher also contributed significantly to his victory.

Several teachers interviewed yesterday described Simons as a leader who had become "comfortable, ineffective and passive."

Gertrude Turner-Wills, a French teacher at Dunbar High School, echoing other teachers, said her vote for Fisher was partially a "vote against Simons. He'd gotten comfortable and taken his position for granted. Had Bill Simons won, I would've gone to the NEA."