Maine: 36 M.R.S. § 1760
36 M.R.S. § 1760. Exemptions.
Subject to the provisions of section 1760-C, no tax on sales, storage or use may be collected upon or in connection with:
1. EXEMPTIONS BY CONSTITUTIONAL PROVISIONS. Sales which this State is prohibited from taxing under the Constitution or laws of the United States or under the Constitution of this State.