The United States originated as Colonies, and the Colonies were separate groupings of people differentiated by culture, politics, religion, ethnicity, etc. The idea of maintaining States (rather than throwing everything into a single Federal stewpot) was explicitly and specifically to retain those differentiations, so that citizens could live in and support a state that best represented them.
I live in rural Upstate New York, and would be very pleased to separate New York City from the rest of the State -- we have nothing in common with those city dwellers.
If Oregon's population is divided so strongly, then it makes perfect sense to split the State, and see if Idaho is willing to absorb the conservative folks in eastern Oregon. It is just harkening back to the original concept of the States.
I think it could be defined even between those within the city limits and outside the city limits. Here in Michigan, the vast majority of the state is deep red, except the highly densely populated blue cities, which carry the politics of the state unfortunately.