The liberal Economic Opportunity Institute (EOI) has filed a legal brief asking to defend Seattle's income tax. EOI says it should be added to the lawsuits because Seattle "does not adequately represent EOI's interests."  What are those interests? EOI explains it is critical to have Seattle's income tax upheld to advance its efforts to "advocate for local income taxes" across the state.

EOI tells the court that it "has been a primary public advocate supporting" the Seattle income tax. Perhaps EOI could have strengthened its argument to the court for why it should be party to the lawsuit by also mentioning it received a $49,500 contract from Seattle to develop the income tax proposal. The main legal argument that EOI is trying to make is that the state law prohibiting a city from enacting an income tax is unconstitutional. So there you have it - this fight has little to do with Seattle and everything with trying to bring an income tax statewide. 

Additional Information
Timeless advice from WA Supreme Court on income taxes
Local Income Taxes Are Illegal in Washington state
Why did legislature pass 1984 local income tax ban?