Washington state's tax system has been called the most regressive in the country, as the state collects no income tax nor do any of its cities. But Seattle is looking to change that: On Monday, its city council passed an income tax on its highest earners.The council voted 9-0 in favor of the tax, which will apply a 2.25 percent tax on annual income over $250,000 on individuals, or $500,000 for couples filing jointly. The city estimates the tax will generate $140 million in new annual revenue."Seattle is challenging this state's antiquated and unsustainable tax structure by passing a progressive income tax," said Mayor Ed Murray in a statement. "Our goal is to replace our regressive tax system with a new formula for fairness, while ensuring Seattle stands up to President Trump's austere budget that cuts transportation, affordable housing, healthcare, and social services. This is a fight for economic stability, equity, and justice."Opponents are already preparing lawsuits to fight the tax. The Freedom Foundation, an Olympia-based conservative think tank, told NPR station KUOW that it was forming a coalition to litigate the tax, which it says is illegal under state law."If the current tax system has problems, then let's address those," Jami Lund, the foundation's senior policy analyst, told the station. "Let's not add an extra injustice to sort of balance another injustice. I would say it's like fighting fire with fire and you end up with all houses getting burnt down."NPR station KNKX reports that legal experts see the city's legislation as a test case that could open the door to other Washington municipalities passing income taxes: