THE US Supreme Court has ruled that people who donate money for scholarships to religious schools are eligible for rebates on their taxes.
The decision gives momentum to people who back government help towards private and religious school fees.
Critics say that the government is subsidising religious schools - a move they claim is a flagrant violation of the legal separation of church and state.
The court allowed the tax credits for religious schools to continue in Arizona, the one state where they are allowed, and where they had been legally challenged.
But the decision could affect other states. The state of Florida, for example, has adopted the nation's first programme where families can receive vouchers to help with private school fees. Wisconsin and Ohio have adopted similar so-called "school choice" measures. Maine also offers tuition assistance for families whose children attend non-religious private schools, a policy that also is the subject of a pending lawsuit.
Right now, "the momentum remains on the side of school choice supporters", said Clint Bolick, vice-president and director of litigation at the pro-voucher Institute for Justice.
The Reverend Barry Lynn of Americans United for Separation of Church and State disagreed. "'It's dangerous to read too much into this," he said, adding: "We hope the court will stop this erosion of the church-state wall of separation."
The Arizona tax credit, which took effect in 1997, allows those who pay tuition fees or make charitable contributions to private or religious schools to deduct $500 (pound;310) from their taxes. About $75 million (pound;47m) has been made available for private school tuition under the programme.
Opponents who include the National School Boards Association, argued in court documents that the tax credits were "a thinly disguised means of transferring public funds to the coffers of private, mostly sectarian, schools".
The highest court in Arizona, however, had previously ruled that the credit did not violate the separation of church and state because it "does not prefer one religion over another, or religion over non religion" but merely widened the range of private choices.
The Supreme Court decision in the Arizona case will not be its last word on the subject. The court is scheduled to hear arguments in December in a dispute over the use of taxpayers' money to supply computers and other materials for religious schools in Louisiana.
What might happen then is not clear, since the justices have previously given mixed signals about public subsidies for private and religious schools - making these decisions all the more important.
In 1973, the Supreme Court struck down a New York law allowing tax back on private nonreligious and religious school fees, saying there was no way of guaranteeing that the subsidy would go toward non-religious purposes.
More recently, however, it has refused to block state vouchers for children of poor families to attend private schools in the midwestern city of Milwaukee; and it also upheld tuition tax credits for religious and non-religious private school fees in Minnesota.