A longtime assistant to Washington Teachers Union President William Simons announced his candidacy yesterday for the job Simons has held since the union was created 20 years ago.

Harold Fisher, 47, who has been described as Simons' right-hand man, was recently fired by the union leader. Simons, he charged "is trying to discredit me . . . to block my candidacy."

Simons, 60, who has successfully defeated the few who challenged his position as head of the 4,500-member union, has not said whether he will run in the May election.

A union spokesman said yesterday that Simons was in Miami on union business and unavailable for comment. Simons did not return a reporter's calls.

In a Jan. 4 letter to Fisher, Simons stated that Fisher had been irresponsible and ordered that he stay away from a Northeast school where he had had several conflicts with the principal. In a Jan. 23 letter, Simons relieved Fisher of all union duties and ordered him to stay away from the union headquarters at 2101 L St. NW.

Fisher is on administrative leave pending the WTU executive board's decision on the firing.

Fisher, who is known as a task-oriented union representative, joined the union's staff when it was founded and has held various positions.

Simons, long unchallenged for the union presidency, has been able to beat back opposition that evolved in recent years.

Simons defeated James D. Ricks, a former D.C. teacher, in a controversial election in 1982, but the results were thrown out after U.S. Department of Labor officials found that secrecy had not been maintained during the balloting at several schools. After two more elections, Simons was officially declared the winner.

At a press conference yesterday, Fisher said he wants to "place the membership back in it rightful position as the governing unit of the organization."

Fisher charged that in recent years Simons, once considerd a firebrand leader, has become unresponsive to union members and "cozy" with the administration and management of the D.C. public schools. He said union morale is low.

If elected, Fisher said he would poll members to determine if they are satisfied with union services and benefits and to solicit ideas for improvement.