The D.C. Court of Appeals, on a 6-to-3 vote, yesterday ordered that the educational tax credit initiative be placed on the city's Nov. 3 ballot, signaling the official start of a campaign that many expect will be the most hard fought this year.

The ruling, made after almost two hours of oral argument, ended several months of legal uncertainty about the referendum. The National Taxpayers Union has mounted a major effort in favor of the proposal, while national labor unions, spearheaded by the American Federation of Teachers, announced plans yesterday to fight its passage.

Under the proposal, city residents could deduct up to $1,200 per child from D.C. income taxes for expenses they incur at either public or private schools.

"I'm happy to see that justice has been done," said Bill Keyes, chairman of the D.C. Committee for Improved Education, which collected over 27,000 signatures for the tax credit referendum. "Our opponents have done everything they could to frustrate the will of the people and keep this off the ballot. Now the people will be able to exercise their right to vote for something that's very important to them, that parents will have the right to decide their children's education."

Wilbert Williams, campaign manager for the D.C. Committee Against the Tuition Tax Credit, said his group was disappointed by the ruling, but anticipated it.

"We weren't expecting it so quickly," said Williams, who is an organizer for the AFL-CIO. "But now we know we'll have three or four weeks to work at this so we'll be out there campaigning . . . We'd prefer having people defeat it on the ballot so the citizens here can send out a message."

Numerous D.C. officials have announced their opposition to the measure, including Mayor Marion Barry, City Council Chairman Arrington Dixon and School Superintendent Floretta D. McKenzie.

William Simons, president of the Washington Teachers Union and one of the main organizers of the antitax credit group, said labor groups might spend up to $200,000 on the campaign. The proponents already have placed advertisements in buses and subway cars supporting the measure.

"We intend to defeat this thing," Simons declared, "and whatever it takes to do it, we'll do. This is a major issue. It's not an educational issue, but rather a measure to destroy the government."

Simons said about 15 staff members from national and local unions already have started to work full-time against the tax credit proposal.

Yesterday's order by the full appellate court overturned a decision by the D.C. Board of Elections and Ethics, which refused in early August to place the referendum on the ballot, ruling that 82 percent of the 27,000 signatures of registered voters on petitions requesting the initiative had been gathered by seven out-of-towners who were not properly registered D.C. voters.

A three-judge panel of the court -- Frank Q. Nebeker and Stanley S. Harris, with Julia Cooper Mack dissenting -- ruled on Sept. 18 that the measure should be placed on the ballot. The majority cited a Board of Elections rule that signatures of voters who otherwise are qualified should not be invalidated by the failure of a circulator of a petition to be properly registered.

The full court last week agreed to hold a hearing on the issue and suspended the order of the three-judge panel, requiring a vote by the full court.

The 6-to-3 ruling placing the measure on the ballot was announced in a brief written order late yesterday afternoon without either a written or oral opinion.

Harris and Nebeker were joined in the majority yesterday by Judges James A. Belson, Catherine B. Kelly, John M. Ferren and John W. Kern III.

Dissenting were Mack, Chief Judge Theodore R. Newman Jr. and Judge William C. Pryor.

During the hearing, Newman sharply questioned H. Richard Mayberry Jr., the lawyer for the tax credit proponents, about the Elections Board decision that proponents had "manipulated" the petition-gathering process so flagrantly that the measure should be barred from the ballot.

Mayberry responded that the board had no evidence that six of the seven signature-gatherers were not city residents when they registered last spring and circulated the petitions. In any event, Mayberry said the proper way to deal with any fraudulent statements that they were residents would be through a criminal prosecution, "not by disenfranchising the 27,415 voters who signed the petitions."

Ferren said during the oral argument that the case hinged for him on the "unequivocal language" of the elections board rule that "the failure of the circulator of . . . a petition to be a registered qualified elector will not invalidate the signature of an otherwise registered qualified elector."

William H. Lewis, counsel to the Elections Board, said the rule did not apply in this case because "the integrity of the initiative process is paramount.

"The whole process has been tainted," Lewis said, "by wholesale manipulation."

Ferren rejoined that it seemed the board was taking a position that "you can decide ad hoc when to apply the rule."

Five members of the D.C. School Board also will be elected on Nov. 3 as will delegates to the D.C. statehood Constitution convention.