The campaign for passage of the proposed D.C. education tax credit entered its final weekend yesterday with few visible signs of popular support, but its opponents were still fretting that overconfidence and low voter turnout in an off-year election could allow the initiative to squeak through.

The brief but bitterly fought contest over the most controversial issue in this year's school board election has been viewed as a referendum of national significance by outside groups on both sides of the issue, who have poured more than $250,000 into the campaign.

"It amazes me that outsiders come along and try to destroy our public schools just at a time we are making progress," the Rev. Ernest R. Gibson, chairman of the D.C. Coalition Against Tuition Tax Credits, said yesterday at a sparsely attended rally at Western Plaza, across Pennsylvania Avenue from the District Building.

Mayor Marion Barry, who also addressed the crowd of between 300 and 400 public school teachers, union organizers, parents and children, said, "They proponents have made our city into a national experiment . . . but we're not going to tolerate it."

But proponents have refused to be labeled outsiders. "It's unfair to characterize this as a Libertarian (Party) proposal," said Bill Keyes, chairman of the D.C. Committee for Improved Education, sponsor of the measure. "I'm no Libertarian. We have Republicans, Democrats and Independents on the committee."

Keyes backed away yesterday from predicting a victory.

"I'd like to smile and say we're going to win and all that, but to be honest with you, I think we've got a tough row to hoe here for the next couple of days," he said.

"We're going up against City Hall. We're going up against all the big-shot politicians," Keyes said during a televised debate yesterday on WDVM-TV (Channel 9). "We're going up against all the rich people who send their kids to private schools and don't want other people to have the ability to exercise that choice."

Most of the conventional political indicators seemed yesterday to point to a defeat for for the tax credit initiative. Public employe labor groups, businessmen, church leaders, private school administrators, civic groups and the D.C. political establishment -- including all 26 of the city's top elected officials -- are united in opposing the measure.

The Most Rev. James A. Hickey, archbishop of Washington, further aided opponents by remaining neutral on the tax credit issue, even though enactment of the proposal would offer considerable financial relief to Catholic families with children in parochial schools.

Early canvassing by both sides suggested the tax credit lacked support even in affluent Northwest Washington, where many parents send their children to private schools.

But it is not clear how well the broad-based vocal opponents will be able to translate their rhetoric into votes in an election where limited voter interest has prompted only about 14 percent of the registered voters to participate in 1977 and 1979.

Barry and City Council Chairman Arrington Dixon told a reporter Friday they were only guardedly optimistic the tax credit would be defeated. Both of the veteran politicians warned against underestimating the strength of the tax credit's proponents.

"A lot of people want to think it's done and over and the initiative has been defeated," Dixon said. "But I think it will be very close. And the undecided will be affected by the TV ads run by the proponents ."

Barry said he did not think the surveys done were scientific enough to be valid indicators of voter sentiment. And this is an election in which a previously undefined constituency with no readily visible spokesmen -- present and potential private school parents with a direct financial interest in the issue -- has been targeted. Many in that group are considered the most dependable voters.

Both sides view the outcome as an important test case of a concept that could fundamentally shift financial resources from public to private schools. Other states have experimented with various, less costly tuition tax credits and deductions -- most of which have been struck down by the courts. Also, Congress is considering a bill to provide a federal education tax credit.

Under the proposal, known as Initiative No. 7, taxpayers would be able to reduce their D.C. income taxes by up to $1,200 per pupil for educational expenses they incur at either private or public schools. It would also permit nonparents to receive tax credits and corporations could write off up to 50 percent of their annual D.C. tax liability by contributing toward educating low-income students.

The $1,200 limit would increase automatically 10 percent each year or, if the City Council approves, by a percentage equal to the rate of inflation. Opponents say this provision would only exacerbate a drain on city revenues they conservatively estimate at $24 million to $38 million a year, based on the assumption that parents of between 20,000 to 32,000 school-age children would take advantage of the credit.

Barry has said the city will be forced to raise property taxes to recapture the lost revenue.

But sponsors of the initiatve, members of a spinoff of the National Taxpayers Union (NTU) sharply dispute the claims that it would mean a massive revenue drain. Last week, they unveiled an analysis, prepared by a Canadian economist and financed by San Francisco-based Libertarian Party research organization that concluded that the city could actually come out ahead financially.

This would happen, they said, if more than 10,000 students leave the public schools for private schools and the city decides to cut its public school expenditures proportionately.

The initiative differs from the various tuition tax credit bills that have been introduced in the Congress because all educational expenses, not just tuition, would be covered.

The NTU and local proponents of the tax credit plan contend that a $1,200 per pupil tax credit, in public and private schools, would restore "freedom of choice" to thousands of low- and middle-income families that would prefer to send their children to private schools but can't afford to now.

Also, proponents say Washington's school system is in terrible shape and that enactment of the tax credit would force public schools to improve their quality of education if they hope to compete with private schools.

The initiative has been spearheaded by the NTU, a Capitol Hill-based lobby which has allied itself with a locally based coalition of parents, Catholic school educators and Libertarian Party activists.

The NTU has contributed at least $125,000 on behalf of the initiative, which has been used in part for two controversial television commercials that began appearing on three Washington-area stations last Monday.

The commercials, one of which features "Mission Impossible" star Greg Morris, depict the city's school system as ineffective and show children allegedly smoking marijuana.

Since the ads began appearing, Morris said he has received a flood of complaints from D.C. residents, including Dixon. He said, in a telephone interview from his Las Vegas home on Friday, that he agreed to film the commercial as a "public service announcement" and did not realize he was taking sides in a heated political dispute.

"I'm not going to apologize for what I did, but I'm not going to defend it," he said. "I was doing this for improving educational facilities. I had not the slightest idea that this was a political thing."

Opponents, particularly public employe labor unions, consider the tax-credit proposal as a direct attack on the very concept of public education and a major drain on city revenues that could lead to layoffs and reductions in services througout the city.

Tax proponent Keyes, appearing in a second TV debate yesterday on the Charlie Rose Show (WRC-Channel 4), became upset during continued questioning about the advertisements and his group's support from outside the city.

When a questioner in the audience contended one commercial was made in Tuscaloosa, Ala., Keyes said, "The issue is the kids in this city. The damn point is not Tuscaloosa." Keyes rose from his seat and cursed the man in the audience, using words the station deleted from the broadcast. Keyes declined to participate in the closing minute of the program.

Labor organizations including the American Federation of Teachers (AFT), the National Education Association (NEA), and the American Federation of State County and Municipal Employees (AFSCME), have pumped at least $100,000 into the campaign against the tax credit, although only a fraction of that has been publicly disclosed. Americans United for the Separation of Church and State have also contributed.

Most of those funds and "in-kind services" have been funneled through three anticredit groups -- the D.C. Coalition for Public Education, Save our City (SOC) and the D.C. Coalition Against Tuition Tax Credits, a two-week-old group formed to coordinate the overall attack.

Union officials contend that their donations of in-kind services, such as temporarily assigning union employes to work on the campaign or mailing campaign literature, do not constitute political contributions under the D.C. campaign finance law.

As a result, most of the $20,000 that the NEA has spent to produce phamphlets and radio commercials that have been used by one of the antitax-credit groups won't show up in financial disclosure statements filed with the city.

In addition, most of the city's church groups have lined up against the proposal and have distributed antitax-credit leaflets and promised to furnish buses to take voters to the polls.

The voter turnout effort by church members and labor organizations will be particularly important in areas of the city like Wards 1, 7 and 8, where opponents say sentiment runs overwhelmingly against the initiative but voter turnout traditionally has been low.

Moreover, the two major gay activist groups, the Gertrude Stein Democratic Club and the Gay Activists Alliance, both urged their members to vote against the initiative.

Jerry Clark, a political organizer for AFSCME, said last week that the opponents would concentrate their voter turnout efforts in 41 key precincts where they say oppositon is running greater than 3 to 1. Those precincts are located in Wards 1, 6, 7 and 8, which will be visited on Tuesday by canvassers, sound trucks and buses.

"We're going to use soundtrucks in areas where if we get them out they'll support our position," said Wilbert Williams, an anticredit organizer on loan from the AFL-CIO. "Some of these people have no motivation to go (to the polls). We literally have to pick them up and take them there.

Voter approval of the initiative by no means would assure its enactment. Opponents have already said they would challenge the constitutionality of the measure in the courts -- a process that could drag on for months or years.

Moreover, Mayor Barry and City Council member John A. Wilson said late last week they would urge the council to repeal or gut the $1,200 tax credit if it is approved on Tuesday.

"There's no question about it; I'd be the first to recommend to the Council that if this initiative passes, let's take it down to a dollar," Barry said.

Under the city's charter, an initiative approved by the voters and certified by the D.C. Board of Elections and Ethics automatically becomes an act that must be transmitted by the City Council to Congress for review.

The act would become law within 30 legislative days unless both the House and Senate vote to disapprove of it.

The City Council technically could amend the measure at any time after the initiative has been certified by the board of elections, according to Bruce French, the council's legislative counsel. However, some council members facing reelection next year might be reluctant to counteract what might be considered the will of the people.