The U.S. Supreme Court held in a 5–4 decision on Monday that the personal income tax system imposed by the state of Maryland, which did not give taxpayers a credit against their county income tax for taxes paid to other states, violates the dormant Commerce Clause (Maryland v. Wynne, No. 13-485 (U.S. 5/18/15)).
Maryland imposes two income taxes on taxpayers living or doing business in the state—one designated a state income tax, and the other designated a “county” tax, but both are state taxes (slip op. at 2). The state gives taxpayers a credit against state income tax for state taxes paid to other states but does not give a credit against county taxes, which can result in double taxation of some income.
Supreme Court invalidates Maryland’s personal income tax structure – Journal of Accountancy