I think a Democrat could win Washington with just King County nowadays. In 2016, only King, San Juan and Jefferson were more Democratic than the state as a whole, and San Juan and Jefferson are very small.
Edit: Missed Whatcom, but it was only very narrowly more Democratic than the state as a whole. San Juan, Jefferson and Whatcom are all also relatively similar, so the right Republican could win all three while losing King by enough to lose statewide.
If Democrats are losing Snohomish they will lose state wide as well
