The Freedom Foundation, a conservative think tank, sued Seattle after its city council passed a measure in July that would place a 2.25 percent income tax on wealthier residents.

The tax was estimated to affect the 9,000, or 2 percent, of the city’s taxpayers – people who make more than $250,000, or $500,000 for couples filing jointly.

The lawsuit, filed on behalf of 19 Seattle citizens, alleges the proposed tax violates the state constitution and restrictions on cities to impose income taxes, The Wall Street Journal reported. Another group called the Opportunity for All Coalition, founded by Seattle venture capitalist Matt McIlwain, filed a lawsuit the same day, the newspaper said.

Washington is one of seven U.S. states without an income tax, and the liberal-leaning state has had a longstanding aversion to taxing income. The legal battle over Seattle’s proposed tax is seen as a litmus test for the how the state’s politics have