Supporters of a D.C. education tax credit have raised more than $114,000 for their referendum campaign, virtually all of it from the National Taxpayers Union, according to new campaign finance reports.

With 2 1/2 weeks to go until the Nov. 3 vote, the reports indicate that the campaign may well become one of the most expensive ever waged in Washington. On Tuesday, a group of labor unions, spearheaded by the American Federation of Teachers, said they would spend up to $200,000 in fighting the measure, which both sides see as a test case of national importance.

"We're trying to achieve something for the benefit of the entire community, giving people more control over where they send their children to school," said James D. Davidson, chairman of the the Taxpayers Union, a nationwide group opposed to high taxes that has its headquarters on Capitol Hill. "Everybody knows the (public) schools here have failed despite a very large expenditure of money."

William Simons, president of the Washington Teachers Union, sharply attacked the large NTU expenditure. "It demonstrates without a doubt," Simons said, "that this endeavor is not locally supported, but rather is supported by people who have other axes to grind rather than improve the educational system of the District of Columbia."

Under the tax credit proposal, taxpayers would be able to reduce their D.C. income taxes by up to $1,200 per pupil for expenses at either private or public schools. It would permit nonparents and corporations to contribute their tax credits toward educating low-income youngsters.

In August, after complaints by city leaders who fear a large revenue drain from the measure, the elections board barred the tax credit initiative from the November ballot. The board ruled that most of the 27,415 signatures on its petitions had been gathered improperly by out-of-towners. On Tuesday the full D.C. Court of Appeals ended months of legal uncertainty by overturning the board's decision and ordering that the measure be placed before the voters.

Teachers Union president Simons has played a key role in putting together a coalition of unions, local political leaders and civic groups to oppose the tax credit drive. He said yesterday that supporters of the proposal were part of a "radical right movement to use 'lawful means' to destroy the government," a contention NTU Davidson leader strongly denied.

Simons said the new anticredit group, called the D.C. Coalition Against the Tuition Tax Credit, will hold a kick-off press conference this morning. He said Mayor Marion Barry, Del. Walter Fauntroy, and City Council Chairman Arrington Dixon would participate.

The group has not yet officially registered or filed any campaign finance reports with the D.C. Board of Elections and Ethics.

According to reports filed with the board by the D.C. Committee for Improved Education, the group that is mounting the tax credit drive, the National Taxpayers Union has contributed $114,091 to the effort since March.

Only $360.63 has been contributed by four local residents, the reports indicate.

"We really couldn't start raising local money until we were sure we'd be on the ballot," said Charles M. Pike Jr., campaign manager for tax credit group. "Now we're out there trying."

Fighting the legal moves to block the referendum has cost the committee almost $18,000 so far, and Pike said the group's total legal bill may reach $30,000.

Two weeks ago the group paid $50,000 to a public relations firm, Totalcom Inc., of Tuscaloosa, Ala., to finance an advertising campaign. Pike said the effort includes ads that have already appeared on Metro buses and subways plus a telephone bank, and television and newspaper ads that will begin running soon.

Until now, the most expensive referendums in Washington have been the two campaigns for legalized gambling which cost slightly over $100,000 apiece last year. Each of the leading candidates for mayor in the 1978 Democratic primary spent between $200,000 and $400,000.