Seattle just took a major step to fight income inequality.

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By a 9-0 vote, the city council on Monday approved a new income tax on the city's wealthiest residents. Mayor Ed Murray, who supports the measure, is expected to sign it into law next week.

"The measure applies a 2.25 percent tax on total income above $250,000 for individuals and above $500,000 for married couples filing their taxes together," the Seattle Times reported.

Republicans are already mounting opposition.

Republicans in Seattle and conservative political organizations in the region say the tax violates the state's constitution, which states that all "property" must be taxed uniformly. They also argue it violates a 1984 state law that prevents cities from taxing income. "Seattle’s income tax proposal is clearly illegal and unconstitutional," the conservative Washington Policy Center wrote in June. Washington does not currently charge any income taxes.

"An income tax that starts out targeting 'the rich' above a certain income level would likely be extended to individuals and families farther down the income scale," Paul Guppy, vice president for research at the Washington Policy Center, told ATTN:. "This has been the common experience of the federal income tax and most state income taxes."

The tax is supposed to raise $140 million per year, and that money will go to lowering property taxes, helping the homeless, building affordable housing and more.

City councilmember Kshama Sawant, who introduced the measure with councilmember Lisa Herbold, told ATTN: the tax is necessary to fight Seattle's growing income inequality problems, and called Washington's laws about income taxes "ambiguous." She said Seattle's tax code is regressive, meaning it takes relatively more income from less wealthy people than it does from the richest residents.

"Washington has the most regressive tax system in the entire nation," Sawant said. "Seattle has a deepening problem with income inequality. It's a booming city, but... you're seeing this city become increasingly unaffordable and unlivable for the vast majority of people, and I don't mean just the poorest people. It's also people who otherwise might have thought they were part of the middle class are now being steadily displaced out of the city. The rents are sky rocketing, and homelessness is on an alarming rise. It is absolutely necessary to push back against this inequality."

Sawant noted that some of the richest people in the world—like Microsoft co-founder Bill Gates and Amazon CEO Jeff Bezos—live in the Seattle are and have their business headquarters in the area.

Sawant, who became one of the first socialists elected to public office in decades in 2013, campaigned on raising Seattle's minimum wage to $15 during her successful city council campaign. Seattle approved raising the minimum wage in 2014.

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"People are sick and tired of the corporate agenda, and people are angry at [President Donald] Trump, the billionaire class, and people want to defeat the Trump agenda," Sawant said. "The lesson from our struggle has been that we need movements that are independent of corporate politics and independent of the politicians who pay lip service to the issues of working people, but are people who at the end of the day are bought and sold by corporations."

Richard Wolff, a professor of economics emeritus at the University of Massachusetts, Amherst, told ATTN: that he also sees the Seattle movement as a fight for the working class.

"The 9-0 vote and mayoral support reflects that such policies are expressions of majority sentiment long blocked or neutered in/by US politics," Wolff said. "This is a logical next step from what Occupy Wall Street began and [presidential candidate] Bernie [Sanders] stregthened."