IT'S JUST the kind of proposal to appeal to Ronald Reagan. First you create a big new federal responsibility to drain billions from the Treasury. Never mind all that talk about a balanced budget, about finally reining in Washington. What you want is a new, uncontrollable revenue loss that doesn't address any national problem.

Then you want to make sure that this initiative intrudes deeply into state authority, that it doesn't chiefly benefit the "truly needy," and that it raises the prosepective threat of more regulation of the private sector. But, above all, you want to make Washington responsible for something long considered none of its affair: subsidizing attachments to religious and ethnic groups.

If you think you couldn't sell this to President Reagan and other newly ascendant conservatives, you are sorely mistaken. They have already bought the package, which goes by the name of "tuition tax credits" for parents of parochial and other private school students. (The tax credit -- a dollar-for-dollar tax reduction -- would also apply to college tuition, a largely seperate question but one that shares in the fiscal folly.)

Indeed, the Reagan support for this measure -- specifically, his pledge last October to push the type of bill reintroduced in the Senate by Republican Bob Packwood of Oregon and Democrat Daniel P. Moynihan of New York -- suggests how far conservatives may stray from their principles.

Consider, for example, that old conservative notion -- even some moderates and liberals, of all people, agree with it -- that Washington should keep its nose out where there is no problem to begin with.

Tuition tax credit supporters of course have long contended that there is indeed a problem, that parochial schools in particular have been facing dramatic enrollment declines, that we are witnessing "the conquest of the private sector by the public sector," as Moynihan is fond of putting it. Packwood even declared during a 1978 Senate floor debate: "Without this tax credit we will see private education in this country shrink to, I will take a guess, 3 percent, 4 percent, 2 percent, I am not sure."

Bad guess.

Private elementary and secondary school enrollments have been doing nicely without any federal tax credits. As the National Center for Educational Statistics reports, the private school share of total enrollments even crept up a fraction over the last decade, from 10.5 percent in 1970 to 10.9 percent in 1980.

Yet Packwood, Moynihan and others still want a tax credit covering half of tuition and fees, up to $500 a year when the measure is fully effective -- and undoubtedly more later. Indeed, Packwod stated in a recent U.S. News & World Report interview: "I want to start with a relatively modest credit to establish the principle."

The estimated cost of this "modest" credit when fully effective is nearly $2 billion a year, just to cover students already enrolled in parochial and other private elementary and secondary chools. That's almost $2 billion -- before including the more expensive higher education portion -- which most Americans would have to make up for by accepting a larger share of the federal tax burden. For openers.

If this heavier tax load for the majority would solve no national problem, it certainly would help establish or advance some intriguing principles -- especially for conservatives.

Start with Washington meddling in state affairs. Even if private school students were deemed to deserve special aid in some localities, the obvious question for any good conservative -- a question President Reagan likes a lot -- is: Is this a federal responsibility? Why?

The states, which are constitutionally responsible for education and its financing, clearly can provide financial breaks for private school students if they wish and if they can fashion legally acceptable means. Many states have had heated debates on this issue. A tuition tax credit petition stirred considerable dispute in California last year, for example, but the plan failed to make it to the ballot. Similarly, a tuition credit bill died in the Wyoming legislature this year.

New York, on the other hand, enacted tuition tax breaks for private school students in 1972, following passage of tuition reimbursements in Pennsylvania the year before. Minnesota, Rhode Island and New Jersey chose tuition tax deductions.

The problem is that almost all of these state laws have been struck down as unconstitutional on church-state grounds. The one surviving today is Minnesota's. But even that measure -- not terribly different from the Rhode Island law that died in federal appeals court last September -- is under court challenge again, and its future is uncertain.

The question is not only why a federal tuition credit wouldn't be struck down by the Supreme Court as well -- and the odds clearly are that it would be -- but why Washington should stretch its hand into this area at all. Carolyn Warner, education chief of Arizona, puts the matter bluntly: "It is either hypocrisy or ignorance or the greatest con job of the century for Washington to think that tax benefits for private school students are any of its business."

Next, consider conservatives supporting a measure that would help establish a favorite principle of their civil rights movement antagonists.

This one stems from the idea that govenment ought to distribute benefits according to one's ethnic identity. That notion, central to the worldwide ethnic political wave, emerged in this country in the 1960s with demands first by blacks and then by American Indians, Hispanic Americans, Asian Americans and, ultimately, white ethnic groups.

White ethnic spokesmen in fact complained loudly at times about not getting their fair share (dare one say "quota"?). In a 1975 speech, for example, the Rev. Andrew M. Greeley, a sociologist at the University of Chicago's National Opinion Research Center, attacked the little Ethnic Heritages Studies program -- created to chiefly to help appease white ethnic groups -- as "tokenism with a vengeance."

With conservatives now getting programs backed by blacks, Hispanic Americans and other racial minorities, it is not difficult to see that tuition tax credits would represent the triumph of the white ethnics.

The overwhelming majority (85 percent) of private elementary and secondary students, after all, attend the religious schools of mainly white ethnic groups. Of these, most (76 percent) are in Catholic schools, which are quite naturally the largest single force behind the tuition tax credit drive.

Tuition tax credit sponsors make no secret of the fact; that parochial school education would be the dominant beneficiary here. Understandably, they don't stress the tax subsidies that would help pay tuitions at the junior watering holes of the wealthy, the Exeters and Andovers. Besides, as Moynihan noted when reintroducing the measure, "the number of such schools is growing" on its own.

No, he said that "the object of tuition tax credits is to precent further decline" at sectarian schools, because, he contended once more, "Their number and enrollments have been declining sharply." (Not so. "Catholic elementary and secondary education has remained at a relatively constant level over the past five years," in the words of the National Catholic Education Association, and enrollments at virtually all other faiths' schools have been rising.)

But the principle here goes beyond just switching benefits to favor mainly white ethnics. It encompasses the idea that Washington has an "affirmative" responsibility to subsidize ties to these groups.

Conservatives have bitterly fought that proposition, most recently when advanced by some Hispanic American and other advocates of bilingual education. President Reagan himself recently declared that "it is absolutely wrong and against American concepts to have a bilingual education program that is . . . dedicated to preserving their native language . . ."

It is actually very American and admirable for ethnic leaders to promote ties to mative languages and cultures, just as it is unquestionably American and admirable for religious leaders (often the same people) to cultivate religious attachments. But the point is that neither is a federal responsibility, that Washington has rejected past pleas for what might be called "affirmative ethnicity," let alone for "affirmative religion."

Now tuition credit forces want aid predominantly for students in schools whose basic reason for existence is to maintain religious ties -- and they have even borrowed some civil rights movement rhetoric for the occasion. Anyone familiar with the literature will recognize their calls for affirmative government support of "pluralism" and "diversity" in education as echoes of the bilingual, bicultural and Black English movements.

If this were not enough, conservatives might ponder the prospect of tuition credits leading to extensive federal regulation of private schools, if not immediately then in the not too distant future.

Harvard sociologist Nathan Glazer, who prefers voucher proposals and hasn't firmly decided about the tuition credit idea, says: "Perhaps the strongest argument against it is that the courts will insist that they [private schools] are really public schools and that they will then lose all of their benefits."

Tuition credit forces badly want to avoid that. The Moynihan-Packwood bill states, for example, that the tax credits for parents would not be assistance to the schools themselves -- an attempt to shield private educators from an array of laws and regulations governing those who get federal aid. But it is an open question how much difference that distinction would make in court.

Courts judge more by a law's effect than by its stated intent. And one of the effects many expect from this measure -- however perverse again for its conservative supporters -- is that it will result in higher tuitions than the schools otherwise could charge, leaving little real benefit for parents.

"I would expect a tax credit to make it easier for all tuition-charging educational institutions to raise tuitions by the amount of the credit, converting it into a form of aid to the institutions rather than to the students' parents," says David Breneman of the Brookings Institution. "It's not very different from the relationship between the availability of mortgage money and housing prices -- when easier financing is available, it contributes to rising housing prices."

What we would have in that case is an ever-growing drain on Treasury as tuitions keep rising and Packwood's "modest" credit keeps expanding to catch up.

This might bring to some minds, especially conservative ones, the relationship between federal rent supplements and rising rental prices, or Medicare and climbing medical costs. In fact, Sheldon Elliot Steinbach, general counsel for the American Council on Education, a higher education umbrella group, fears that some lawmaker might start thinking about precisely those things.Steinbach, who was a member of the Reagan campaign's education team, says:

"If tuition tax credits pass and university tuitions or other fees go up next year -- as they are almost bound to due to inflationary pressures -- it's likely that some member of Congress will start a movement to control tuition and room-and-board fees at colleges and universities. Look at hospital cost containment proposals -- you'll have education cost containment."

Whatever courts or Congress might decide, the more immediate regulatory issue would involve Internal Revenue Service authority over the tax-exempt schools, especially to ensure nondiscrimination.

It is unclear how much IRS enforcement of civil rights there really might be. For one thing, the bill would permit exclusion from private schools by religion, which should be no surprise: Most parochial schools by their nature divide students according to religion.

Some of these schools certainly accept students of other faiths. Catholic schools, for example, are 91 percent Catholic, 9 percent non-Catholic, largely because of more non-Catholic black enrollments. But this can create sensitive questions regarding religious instruction for non-Catholics.

Chicago's Holy Angels School, for example, requires both these children and their parents to be instructed in Catholicism, and the Rev. Paul B. Smith, the principal, reports "about 130 to 150 conversions a year in the past" and "about 80 or so conversions last year." More typically, says political scientist Thomas Vitullo-Martin, Catholic schools "tend to adapt their religious instruction when they contain significant numbers of non-Catholics, such as the all-Chinese Catholic school in Philadelphia that honors Taoist rituals and traditions along with its Catholic instruction."

Many parochial school leaders, however, do not want to be forced to admit students of other faiths, and the tax credit bill certainly would permit such exclusion.

For another thing, it's questionable whether Congress would let IRS investigate racial discrimination. The lawmakers have already blocked IRS attempts to get at segregated academies, and one could expect the same today, though Congress' conservative complexion could well change in the future. Moynihan and Packwood, in any case, are in a strange position on this score: While seeking to avoid the regulators' reach, they say their bill has "strong" language on civil rights -- the source of most regulatory battles.

The only thing one can count on here are those court fights. Says civil rights attorney Joseph L. Rauh Jr.: "If tuition tax credits ever became a reality, civil rights groups would fight them in the courts to the last ditch. It might even provide an opportunity for us to get the segregationist academies."

Finally, conservatives might consider the fact that their concern for the "truly needy," historically shaky to begin with, would virtually disappear with the tuition tax credit plan.

Reagan, it will be remembered, is pressing for a 25 percent cut in federal school aid, which mainly provides extra help for poor, handicapped and otherwise disadvantaged students -- including those at private schools. Moynihan has long been right in complaining that needy parochial school students have been shortchanged under those programs. That abuse, addressed by congressional reforms in 1978, was unconscionable.

But it's certainly no excuse for Congress to take money away from the "truly needy," transfer it chiefly to the better off through tuition tax credits, and make no difference in anybody's education in the process.

It should surprise nobody that private school students are generally more affluent than public school children. Sociologist James Coleman's controversial new study of private high schools, for example, shows that families with incomes above $38,000 a year are more than four times as likely to send their children to the private schools than families with incomes under $7,000.

What may raise some eyebrows, though, is the fact that a tuition tax credit is unlikely to result in many, if any, more students attending private schools. To begin with, there's a "supply-side" problem -- many Catholic and other private-school classrooms, already bulging, couldn't accommodate more students. Then there are the expected tuition increases that would offset the tax benefit.

But most of all -- and contrary to popular perceptions -- there's evidently no desire among the population to rush to private schools. Coleman's study, for example, assumed that everyone was simply given an extra $1,000, and that there were no tuition increases and plenty of classroom seats available. How much effect did he project among the whites, blacks and Hispanic Americans he studied? Almost none.

"Only a very small proportion of public school students would shift, less than half of 1 percent of any of the three groups," he found. So much for what migh remain of arguments about the need for "choice," "competition," "diversity," more integration and more educational opportunity for the disadvantaged.

The reality is that tuition tax credits -- at least in their early and "modest" stage -- would merely force the overwhelming majority of Americans to subsidize a couple of billion dollars in tax breaks for a largely better-off minority already in parochial and other private schools. And, judging by the available evidence, that would essentially remain true even if the size of the credit and the resulting federal budget drain were increased significantly.

The same pattern of giving aid mostly to the non-needy holds at the higher education level. Though the Reagan administration wants to cut college grants and subsidized loans for parents who don't really require them, the college tuition credit when fully effective would simply spread around an estimated $2.5 billion more a year indiscriminately -- with no regard for family need.

In short, perhaps the most remarkable thing about the tuition tax credit plan is that conservatives can manage to keep a straight face while supporting it.