An association of Christian schools and a Missoula family are challenging a Montana rule that excludes religious schools from a program created to provide scholarships for private-school students.

The Pacific Legal Foundation said Tuesday that it filed a federal lawsuit challenging a Revenue Department rule for implementing a tax credit of up to $150 for donations made to private schools or to innovating programs at public schools.

The rule, adopted on Dec. 15, bars religious schools from benefiting. It cites the Montana Constitution's prohibition on appropriations to religious institutions.

The federal lawsuit filed Monday by the Association of Christian Schools International and by Kathy and Jerry Armstrong of Missoula, argues the regulation violates the U.S. Constitution's guarantees of religious freedom and equal protection.

A state lawsuit challenging the rule was filed on Dec. 16.