The WSJ editorial writers have nailed it. 

A great beauty of the American federal system is that any of the 50 states can offer its policies as an experiment for others. So the nation owes some gratitude to Oregon for testing whether it is possible for a state to tax its way from deep recession to prosperity.
The battle in Oregon is a case study in the political drama now unfolding in many states. Essentially, it’s about whether a state’s wealth belongs to its public employee unions or to everyone.
The public unions are the primary drivers behind the Oregon tax hike campaign. In recent weeks, national powerhouses AFSCME and the SEIU have poured close to $1 million into the state campaign to secure passage. Oregon’s public employees have one of the sweetest deals in America. Their average pay is about one-third higher than that of private Oregon workers, and Oregon public employees don’t have to pay anything toward their health-care benefits.

…the current tax proposal imposes a first-ever “gross receipts tax” on certain retail and wholesalers. This is a disguised sales tax.