If you are going to win this, it is going to be a matter of "positioning" the issue.
We tend to think of this as only a second amendment issue (and I don't disagree with this), but it risks appealing to only a specific audience.
In a more liberal state like WA, I think part of the key will be to position it among the "rights of the individual", not just "rights to bear arms".
Somehow, you need to associate and connect the idea of gun ownership with "rights" that the left side of the political spectrum can understand.
For example, that segment tends to be fairly pro - marry anyone and pro-abortion. I know it is a big step for many people, but even if you don't believe in those things, if you can somehow associate the idea that "gun rights" are just another "right of the individual like - marry who they want, etc."
I am not saying that I promote those concepts, I am just saying that if you want to keep the segment of voters from "taking away our rights", then they need to see that this law is the equivalent of people wanting to take away the rights that they perceive as important.
When I visited Seattle a few years ago, I was actually pretty surprised how extreme some people there actually were, but it was more a mob mentality than based on real knowledge. We see this around here in the San Francisco area all the time.
What seemed to work for me, was to go hang out in the coffee shop (or bar + grill) and just casually engage people who came in, ask what they were up to, and just start talking about how concerned you are that politicians are trying to take away "peoples rights", like voting rights, marriage rights, seizing private property, and even basic things like how you use a gun a the range.
Get them to think about the typical gun owner as no different than someone who plays golf. Peaceful, working on self-improvement, a hobby. That way they think of it as more of a left wing - white collar activity rather than something that strange people do.
What matters is getting the right laws passed, and the wrong ones dropped, not if the person voting has some other non ideal social views.