Me. Stat. tit. 36 § 5217-A
A resident individual is allowed a credit against the tax otherwise due under this Part, excluding the tax imposed by section 5203-C, for the amount of income tax imposed on that individual for the taxable year by another state of the United States, a political subdivision of any such state, the District of Columbia or any political subdivision of a foreign country that is analogous to a state of the United States with respect to income subject to tax under this Part that is derived from sources in that taxing jurisdiction. In determining whether income is derived from sources in another jurisdiction, the assessor may not employ the law of the other jurisdiction but shall instead assume that a statute equivalent to section 5142 applies in that jurisdiction. The credit, for any of the specified taxing jurisdictions, may not exceed the proportion of the tax otherwise due under this Part, excluding the tax imposed by section 5203-C, that the amount of the taxpayer's Maine adjusted gross income derived from sources in that taxing jurisdiction bears to the taxpayer's entire Maine adjusted gross income; except that, when a credit is claimed for taxes paid to both a state and a political subdivision of a state, the total credit allowable for those taxes does not exceed the proportion of the tax otherwise due under this Part, excluding the tax imposed by section 5203-C, that the amount of the taxpayer's Maine adjusted gross income derived from sources in the other state bears to the taxpayer's entire Maine adjusted gross income. [2003, c. 673, Pt. JJ, §4(AMD); 2003, c. 673, Pt. JJ, §6(AFF).]
36 M.R.S. § 5217-A