The tax, passed by the Seattle City Council last month, targets high-income earners as part of what local lawmakers describe as "a new formula for fairness."
The tax measure requires residents to pay a 2.25 percent tax if they are a single filer and make more than $250,000 annually or file jointly and make more than $500,000.
Its passage prompted a court challenge from the Freedom Foundation, a conservative think tank that considers the tax a slippery slope that could open the door to more taxes in the future.
The outrage over the tax even prompted the Washington Republican Party to call for "civil disobedience" and urged its members to "refuse to comply, file or pay."
Wednesday's suit, filed in King County Superior Court on behalf of several of the city's residents, lays out the case against the tax.
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