A conservative group is suing the city of Seattle over its controversial income tax that targets high-income earners as part of what local lawmakers describe as "a new formula for fairness."

Seattle's tax code, passed by its city council in July, now requires residents to pay a 2.25 percent income tax if they are a single filer and make over $250,000 annually or file jointly and make above $500,000. Washington is among one of seven states nationwide that does not collect a personal income tax, so the city's tax is the only income-based tax Seattle residents might pay.

Passage of the tax has been met with a legal challenge from the Freedom Foundation, a conservative tank that calls the tax an "assault" on the law that sets a dangerous precedent.

"This is clearly bad policy and illegal, but it's also an assault on the rule of law," David Dewhirst, a lawyer for the Freedom Foundation, told Fox News. "If they can get away with it this time, where does it stop?"

Washington's Republican Party has also called for "civil disobedience," and is urging its members to "refuse to comply, file or pay." The lawsuit was first reported by Fox News.

Wednesday's suit on behalf of several of the city's residents argued that the city's plan to tax the rich is unconstitutional, because the state of Washington already imposes strict tax levies, forbids taxes on net income; and requires cities to get permission to tax residents.

The Freedom Foundation is accusing the city of willfully and knowingly adopting "a law that can only survive if the courts abandon decades of precedent – precedent grounded in Washington's fundamental commitment to legal equality."

Seattle's outgoing Democratic mayor, Ed Murray, told Fox News the goal of the tax "is to replace our regressive tax system with a new formula for fairness while ensuring Seattle stands up President Trump's austere budget that cuts transportation, affordable housing, healthcare and social services."

According to estimates, the new tax could generate $140 million a year and cost between $10 million and $13 million to set up, with an additional $6 million a year in enforcement costs. Funds generated are allocated for affordable housing projects and social services for low-wage workers.