HE MOVEMENT to give private schools large federal subsides, through tax credits, is picking up momentum. In the Senate, the tuition tax-credit bill has 49 sponsors. We recently observed in this space that the bill is a threat to the American public-school tradition. Of the many letters that we have received from readers, the great majority vigorously favor government aid to private schools. This bill, and the logic behind it, require a much closer examination than either is currently getting.

Diversity has a great value to education justifying much more direct federal support to private schools, argues Sen. Bob Packwood (R-Ore), one of the authors of the bill. But there is a flaw in that argument - for the greater the diversity among schools, the less diversity each child will see in the the classroom where it counts. Given a range of schools from which to choose, most parents will pick one that is used by other families like themselves. If the tax credit for tuition were actually to go into effect, taxpapers would see, as they went around town, a great and growing diversity of schools - representing many religions, educational theories, national heritages and even language groups. But inside those schools, each child would find himself or herself surrounded by children from similar homes to a far greater degree than in the public school down the block.

No parents are ever fully satisfied with their public schools, because those schools work with too many children ever to address themselves wholly to any one kind or class. But that is a basic lesson of vast importance to young citizens of a democracy. They are learning not only to read, but also to get along with a range of quite different kids as they pursue a common purpose. Other countries use other institutions to nourish this sense of the wider national community - most commonly, the army. We prefer to rely on a public-school system that is never mandatory but, as it present, used by the vast majority of families.

Sen. Daniel P. Moynihan (D-N.Y.), writing in this newspaper last Sunday, described the public-school system in terms of a commercial monopoly. That accusation recalls the debate over the education voucher plan that became very fashionable, briefly, a decade ago. The idea was to give children vouchers to pay for their tuition at whatever school they or their parents might choose. There was a fundamental reason, hard as rock, why the proposal never got far. The easiest students to teach are the children of educated parents. Expansion of the private schools would skim out disproportionate numbers of the youngsters who get a lot of support and direction from their parents. That could only increase the isolation of the other children, those born into less fortunate families. If you think that social class should be hereditary, you will find nothing wrong with this proposal. But most Americans will put themselves on the other side of that argument.

The courts have generally prohibited direct government subsidies to elementary and secondary schools. In one wild flight of fancy, Sen. Moynihan compared this rule with racial segregation. That is precisely wrong. The evil in racial segregation of the schools was that it kept black children out of a central institution of American life. Tuition credits are wrong because they pay families to take their children out of that same institution.

Some senators may possibly be tempted to regard passage of this bill as meaningless, on grounds that the courts will declare it unconstitutional. That's anything but a safe assumption. The tuition tax-credit bill would permit a family to take a tax credit, up to $500 a year, for half of a child's tuition. A tax credit is the same thing as a cash grant in every respect except the legal form. But the courts have always viewed tax benefits through a different lens from that through which they view other kinds of subsidy. Private schools today have unchallenged exemptions from real-estate taxes. Donations to private schools are already a deduction on the giver's federal income-tax return. What the Supreme Court might finally say about a tax credit for tuition is impossible to guess with any certainty. If Congress proceeds with this dangerous bill, it cannot rely upon the judiciary to rescue it from its folly.

The tuition tax-credit bill represents a fundamental departure from a tradition that has proved immensely beneficial to this country. The longer the campaign for the bill continues, the more divisive it will become. President Carter would do a great service by declaring now that, if it is passed, he will veto it.