IT IS NOT UNUSUAL for leaders of Catholic, Protestant and Jewish groups to take the same position on a matter of constitutional law, and not surprising that the government -- in this case the Internal Revenue Service -- agrees with that view. But when Americans United for Separation of Church and State says that both the churches and the state are right, those who challenge this unified view face a formidable array of opponents. This is the situation confronting a group called Abortion Rights Mobilization, an arm of the National Organization for Women, which has filed suit contesting the tax-exempt status of the Catholic Church. The plaintiffs allege that the church's activities in opposition to abortion constitute lobbying, which tax-exempt organizations are not allowed to do.

The case has been in litigation for seven years, and the parties are still tied up in procedural questions. On Monday, the Supreme Court agreed to hear one of them. Ordinarily, individual citizens cannot go to court to challenge the tax-exempt status of other individuals or groups. Courts have held that black parents in the South, for example, had no standing to sue the government in order to deny this privilege to segregated private academies. Nevertheless, a U.S. district court in New York allowed ARM to proceed with this challenge and to subpoena a massive number of documents dealing with church activities, decision-making and communications with members. The subpoena asks for office files on all church programs opposing abortion, copies of sermons and church bulletins -- there are 19,000 parishes nationwide -- and various documents concerning the alleged political activities of the U.S. Catholic Conference. Because the conference is not a party to the litigation -- the IRS is the defendant -- the only way the church group could contest ARM's standing to sue was to refuse to comply with the subpoena and be cited for contempt.

The Supreme Court can end this litigation by deciding the procedural question in favor of the conference. The justices could say that the conference need not honor the subpoena because ARM has no standing to bring this suit in the first place. If the suit is not resolved on these procedural grounds, however, it will become one of the most interesting and complex constitutional cases of the decade.

Should any individual taxpayer be able to challenge the tax status of any other citizen or group and to subpoena private records in the course of that suit? Should churches have some special protection against this kind of court-ordered interference? At what point does a church's teaching on moral questions -- teaching that is directed to parishioners who are voters -- become lobbying? What rights do church leaders have to try to influence elections in which questions of public morality are at issue? Can the government, in effect, penalize this conduct by denying tax-exempt status to religious institutions? If the courts reach these questions, the resulting decisions will surely be as controversial as those involving school prayer and as far-reaching as any tax case in recent memory.