The eastern side of the Cascades in much like Idaho, so it's more a type of Western conservatism rather than a Southern type. It's really a similar sort of divide that you see in a lot of states, which is that urban areas are more liberal and the outer rural areas are conservative. It also has a lot to do with how the states are developed; The western side is a lot more favorable to development.
Exactly. There's also a lot of self-sorting going on. Liberals are moving to the western parts of Washington and Oregon, while conservatives are moving to the eastern parts.
Western Washington also used to be home to a lot of unionized industrial jobs and is now home to a lot of young tech workers.