Plaintiffs-respondents, Arizona taxpayers, filed suit in federal court against the Director of Arizona's Department of Revenue (Director) seeking to enjoin the operation of Ariz. Rev. Stat. Ann. §43-1089 on Establishment Clause grounds. Arizona's law authorizes an income-tax credit for payments to nonprofit "school tuition organizations" (STOs) that award scholarships to students in private elementary or secondary schools. Section 43-1089 provides that STOs may not designate schools that "discriminate on the basis of race, color, handicap, familial status or national origin," §43-1089(F), but does not preclude STOs from designating schools that provide religious instruction or give religion-based admissions preferences. The District Court granted the Director's motion to dismiss on the ground that the Tax Injunction Act (TIA), 28 U. S. C. §1341, barred the suit. The TIA prohibits lower federal courts from restraining "the assessment, levy or collection of any tax under State law where a plain, speedy and efficient remedy may be had in the courts of such State." The Ninth Circuit reversed, holding that the TIA does not bar federal-court actions challenging state tax credits.

Held:

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