Supporters of the D.C. education tax credit initiative released a detailed analysis of their proposal yesterday, contending the maximum possible revenue loss to the city would be far less than what their opponants estimate.

If more than 10,000 students switch from public to private schools, the study says, the District government would actually come out ahead financially.

The 17-page study was prepared by E.G. West, an economics professor at Carleton University in Ottawa, Canada, who has written extensively on American school finance. West testified before Congress last summer on behalf of federal tax credit legislation to aid private schools.

According to his new study, if all D.C. students now in private schools took the credit, the city would lose about $20 million in income taxes. Each additional student who shifts to a private school would be a potential net savings to the city, West said, because the maximum $1,200 per pupil tax credit is far less than half of the $3,133 that the city public schools spend on each student, according to data from the U.S. Department of Education.

If public school expenditures were cut proportionately to the number of students who leave, West said, it would take a switch of about 10,000 students for savings and revenue loss to balance out.

That would represent about a 10 percent drop in the current public school enrollment of about 95,000 and a 50 percent increase in the 20,000 D.C. students now in private schools, a change that West argued could take place in just a few years after the credit went into effect.

In mid-October, Mayor Marion Barry estimated the city treasury would lose $38 million to $76 million if the tax credit passes. He said property taxes would have to be raised steeply to make up the gap. The main coalition against the tax credit has estimated the loss at up to $125 million after three years.

David W. Breneman, at economist at the Brookings Institution who opposes the credit, said West had committed the "fallacy" of assuming that savings from an enrollment drop would be the same as current average per pupil costs. "There are lots of fixed costs (in the public school system)," Breneman said. "It's just spurious and misleading to people to say the city would get anything like the savings he projects."

The initiative, which will appear on the ballot in Tuesday's election, has been sponsored by the D.C. Committee for Improved Education, a local off-shoot of the National Taxpayers Union. Kent Guida, a spokesman for the group, said the West study was financed by the Cato Institute, a San Francisco-based research organization generally aligned with the Libertarian Party.

In his study, West said that virtually all the students who might use the proposed D.C. tax credit to leave public schools would be black because blacks now make up 95 percent of public school enrollment."

Over the past decade, according to D.C. school board figures, the school system lost 43,746 black students and 2,885 whites.

After reaching a low-point in 1976, total private school enrollment by D.C. students increased by 3,601, or 22 percent, in the next four years. Public school enrollment fell by 26,683, or 21 percent, during the same period.

Last year, 53 percent of the city's private school youngsters were black. However, just 10 percent of the total population of black children was in private schools compared to 71 percent of the much smaller population of whites.