Washington is a blue state, hence the reason why they would have more Democrats than Republicans.
You wouldn't expect a poll of Kansas or Utah to have equal numbers of Dems and Reps, so why would you expect one of Washington to do so? Party ID is a changing statistic within groups of people; you can't expect every state to have an equal number of partisans from each side.
By the way, for comparison the last Survey USA poll here which had Cantwell up 17 had 41 percent Rats and 31 percent Republicans.
It's a red state with a blue city that controls the elections.
You're not from around here, are you?
Washington is nearly as Red as it is Blue. It's very closely divided.
In 2004, after two recounts and a bit of fraud the Democrat candidate 'won'.
In 2000, Maria Cantwell 'won' by an extremely thin margin. And now many believe that she 'won' due to fraud from King County (Seattle).
In 2006, Mike McGavick has the issues, the momentum and the money needed to win.