D.C. Mayor Marion Barry said yesterday the city would have to raise property taxes by 20 to 40 percent if voters approve an education tax credit initiative in the Nov. 3 election. He promised to "mobilize the D.C. government" in a major effort to defeat the measure.

"We want people to know that a vote for this initiative is a vote for a tax increase," Barry declared at a press conference called to announce a new push by city leaders, labor groups and civic organizations opposed to the tax credit plan. "You can't have a raid of this magnitude on the city treasury and lose $38 million to $76 million without it being replaced from some other place," Barry declared.

The mayor said he had told his top administrators to have their supervisors pass out information sheets prepared by tax credit opponents and urge their employes "to come out to vote 'no.' "

"We make no bones about it," Barry said. "We will mobilize the D.C. government and do all we can to defeat this ill-conceived proposal."

Backers of the initiative quickly challenged the mayor's figures on the need for a tax increase if the measure passes. Bill Keyes, chairman of the D.C. Committee for Improved Education, accused Barry of using "scare tactics" and said his effort to mobilize city workers against the measure is "improper if not illegal."

Barry revenue advisers said yesterday they based their $38- to-76 million tax drain estimate on the assumption that residents would use tax credits for one-quarter to one-half of the city's school-age children, which they put at 127,000.

At the press conference, D.C. congressional Delegate Walter Fauntroy and City Council Chairman Arrington Dixon joined the mayor in attacking the initiative, which would allow city residents to deduct up to $1,200 per pupil from D.C. income taxes for expenses at either private or public schools.

Thirty-one local educational, religious, labor and civil rights groups and 43 national organizations also announced their opposition to the tax credit and joined a new coalition against it organized chiefly by labor unions led by the American Federation of Teachers.

But Keyes, whose committee for the tax credit gathered more than 27,000 signatures in its referendum petition drive, dismissed the significance of the new group. "If they had spent their time and energy to educate young people there would be no need for an initiative of this type," he said. "A lot of people want to have more control over their children's education."

Keyes' committee is a local off-shoot of the National Taxpayers Union, a nationwide group opposed to high taxes that so far has contributed $114,000 for the referendum campaign. On Tuesday, labor unions, including the AFT and the AFL-CIO, said they would spend up to $200,000 in fighting the measure.

The Rev. Ernest Gibson, chairman of the new D.C. Coalition Against Tuition Tax Credits, said yesterday his group planned to spend $50,000 in a 2 1/2-week campaign that would include advertising, phone banks, and a mass rally on Halloween.

Leaders of the drive against the tax credit indicated that their campaign was starting late because of months of uncertainty about whether the measure would get on the ballot. In August the D.C. Board of Elections and Ethics, acting on complaints by Dixon and others, refused to approve the referendum. The board ruled that most of the 27,415 signatures on its petitions had been gathered improperly by out-of-towners.

On Tuesday the D.C. Court of Appeals overturned the board's decision and ordered that the measure be placed before the voters.

Under the measure, the $1,200 tax credit would be available not only to parents but also to nonparents and corporations who contribute toward educating low-income children.

Because the money would be available from so many sources, Carolyn Smith, director of the D.C. Department of Finance and Revenue, said the total potential drain from the city treasury was $152 million. If credits were used on behalf of one-quarter of the city's school-age children, the revenue loss would be $38 million, Smith estimated. If half used it, the loss would be $76 million, she said.

Smith said the estimates were based on a school-age population of 127,000.

However, according to the D.C. school system, the public schools have 95,000 children this fall with another 20,000 D.C. children attending private schools both inside and outside the city. For these 115,000 children the total potential tax loss would be $138 million.

In the current fiscal year the city expects to raise $298 million from property taxes and $342 million from income taxes, the two largest local sources for its $1.9 billion budget, Smith said.

Barry said his estimate of a 20 to 40 percent increase in property taxes was based on boosting this tax to make up all the revenue lost because of the education tax credit. An aide said the estimate came from a fact sheet prepared by tax credit opponents.

Using Smith's figures on expected revenue, however, the needed increase would be from 12.8 percent to 25.5 percent of property taxes.

Barry press spokesman Edward Myers said last night, "No one knows with precision how much will be lost because of the tax credit. Regardless of the amount, the mayor feels every cent will have to be made up by the public through increased taxes."

Keyes, of the tax credit committee, said the estimates by tax credit opponents of city revenue losses "have ranged from $20 million to $300 million. They come up with all those figures for partisan purposes."

He said an independent estimate prepared by the Media Institute put the revenue loss at no more than $15.5 million because only the credits taken for children currently in private schools represent an actual loss of funds from public schooling. Many of them will not use the full $1,200 credit, he said, because tuition at most Catholic elementary schools is far less.

If more children switch from public to private schools, Keyes said, the city would actually have a net gain because the $1,200 credit each might take is far less than what it costs to educate them in public schools.