President Reagan's determined efforts to enhance the private sector and reduce the cost of government is in many areas supportable. But the American people should question carrying over this philosophy into the field of public education. Proposed legislation to provide tuition tax credits for families sending their children to parochial and other private schools--expected to be announced by Mr. Reagan this week--is flawed on several counts. It is wrong on constitutional grounds. It is wrong as educational policy. It is wrong as economic policy.
* The deepest issue at stake is a constitutional one. The First Amendment enshrines the principle of separation of church and state--a principle that has long safeguarded religious liberty and could be violated only at risk to that precious freedom. This is not airy theory. The Supreme Court has a solid history of striking down laws which directly or indirectly advance a religion or challenge the separation principle. Inasmuch as 90 percent of private schools are church-related, the adoption of tax credits to help parents finance education in such schools is bound to lead to further judicial challenge.
* Such legislation would do damage to public education. First of all, because it would signal federal disinterest in the improvement of the public school system. That many parents want to send their children to independent schools in the face of drugs, vandalism, poor teaching, and other problems at public schools is understandable. But surely it is self-defeating--and uncharitable--to pull the rug of moral support out from under the public schools at the very time many are struggling for their life. Is the Reagan administration in effect saying it does not regard public schooling as important to instilling the values of democracy, racial tolerance, a common culture, and a sense of community?
Under Mr. Reagan's draft legislation, the tuition tax credit would not be large to begin with, but even establishing the ''principle'' of such credits (as one official suggests is the administration's goal) would be unfortunate. And if a gradually phased-in maximum credit of $500 were to go into effect, the federal government would be providing more aid for private than for public schools--hardly a vote of confidence in a system which continues to serve the vast majority of children well. Instead of weakening the public schools, the federal government ought to be helping communities find ways of strengthening them.
It is self-evident that parents have a right to choose religious or other private schooling for their children. But this should not absolve them of the obligation to support those public services, including education, which foster a well-functioning, fair, and humane society - a society which guarantees that very freedom of choice. Parents who send their children to public schools, in turn, should not be penalized by having to subsidize voluntary private education as well.
What does this say, moreover, to America's poor who already face many slashes in welfare, job training, and other benefits and who cannot afford the luxury of private education even with tax credits? If even public schools are short-changed by the administration in the interests of spurring competition in education, where are they to turn?
* Lastly, such tax credits would add to an already exploding budget deficit--an estimated $1.5 billion by end of the three-year period. It is ironic that at a time of fiscal austerity the administration should be supporting another ''tax expenditure.'' Furthermore, if tax credits are adopted, it is likely that many independent schools will take advantage of them to hike their tuition even further. Why invite such an inflationary temptation?
No, the idea is dubious, apparently being given momentum now by political considerations. There is concern in the White House about Mr. Reagan's loss of support among middle-income voters, and it is not without significance that he plans to announce the tax credit legislation in a speech to the National Catholic Education Association. Mr. Reagan no doubt believes he is promoting family interests and religious values. But he should know that many Americans will be troubled by what looks to be a presidential tilting on the side of sectarianism. That would be a high price to pay for political votes.