"According to the Washington State Supreme Court (1951): “It is no longer subject to question in this court that income is property.” This common-sense finding that income is property is important because this means an income tax would need to follow the constitutional restrictions for property taxes. This means, as the state Supreme Court has repeatedly ruled, that a graduated income tax is unconstitutional since property must be taxed uniformly and at no more than one percent of its value."

2017
https://www.washingtonpolicy.org/publications/detail/local-income-taxes-are-illegal-in-washington-state