Yeah, but this year is different. They have an unpopular outgoing Democrat governor, a Democrat president running for reelection, and the fact that Dems have controlled the governor's office there for 28 years. That alone hurts their chances of holding on. McKenna's lead may have dropped recently, but I think that it's temporary and he will ultimately win it all.
I don't see how Democratic coattails would hurt a Democratic candidate. Your first and third points, however, are both valid and good points. Age might be a factor as well; youth is more synonymous with change.
Still, the Democratic lean is strong. If Inslee runs a exemplary campaign he could win in a nail biter.
