Vowing not to repeat what its president said was the "tragic mistake" of endorsing conservative Marian Greenblatt for the Board of Education four years ago, Montgomery County's largest union of school employes is supporting four opponents of the conservative board majority in this year's election.

Leaders of the 4,800-member Montgomery County Education Association voted 105 to 10 Wednesday to support four candidates critical of the board's policies. The four challengers have formed an alliance backed by a 500-member, bipartisan political action committee called EDPAC.

"We made a tragic mistake in judgment with that endorsement," said MCEA President Dave Eberly, also a member of EDPAC's board of directors. "We believe that the current board has not been receptive to the interests or ideas of teachers," he said, adding that he believed a majority of the county's teachers are against the board.

Since 1976, only five of 11 candidates endorsed by MCEA have won, but Greenblatt was one of them.

Fifteen candidates are vying for eight spots in the Sept. 14 primary, from which voters will pick four board members in November. Two members of the present board, Joseph Barse and Carol Wallace, are seeking reelection. Two other incumbents, Greenblatt and Elizabeth Spencer, are squaring off for the Republican nomination for Congress from the 8th Congressional District.

Members of the EDPAC slate are Marilyn Praisner, a CIA analyst; Robert Shoenberg, a University of Maryland dean; Jim Cronin, a Montgomery College professor, and Odessa Shannon, an administrator with the Equal Employment Opportunity Commission. Shannon, the only black running for the board, has raised more money than any other candidate, accounting for $9,472 of the $21,119 raised by the four members of the slate.

By contrast, incumbent Barse and his slate, which also includes Herbert Grossman, a Nuclear Regulatory Commission judge, and Elizabeth Witzgall, a Montgomery College professor, have raised $6,566. Incumbent Wallace has raised $5,645 for her independent campaign.

A recent campaign release from the Barse slate contained news accounts of EDPAC's efforts and said, "Dear Friend: They are organizing to defeat us." The Barse letter said the challengers are part of "a far out clique" who will destroy "traditional education" in the county.

Barse said he was not concerned, however, that the challengers have been more successful in fund raising. His slate is also counting on the fact that most of the 15 candidates are critical of several board policies and that votes will be divided. "Our opponents are split 10 different ways. We are a more concentrated group," he said.

The election highlights a turbulent year of school closings, an unprecedented reversal of three related local board decisions by the Maryland Board of Education, and other closing appeals that still have not been decided. Opponents of board policy say the board has been insensitive to minorities and communities. Supporters say test scores have risen and that the school-closing policy will save thousands of dollars for educational programs.

Also challenging is a slate called "Moderates for Montgomery County," which is made up of Barry Klein, loser by a narrow margin in 1978; Tim O'Shea, a Westinghouse executive, and Vicki Rafel, a PTA activist.

Other candidates are Zoe Lefkowitz, former president of the Montgomery County Council of Parent-Teacher Associations (MCCPTA); Barrie Ciliberti, a former County Council candidate; Sharon DiFonzo, a former MCCPTA second vice president, and Nancy Dacek, a former MCCPTA area vice president.

The 300-member Montgomery County Federation of Teachers, a rival of the MCEA, has endorsed Barse, Wallace, Klein and Praisner.

The 3,500-member Montgomery County Council of Supporting Services Employees (MCCSSE), the union that represents everyone from school bus drivers to office staff, has endorsed the four EDPAC candidates, the Klein-O'Shea-Rafel slate, and Lefkowitz.