...the vote going Republican would have been considered "blue" (in terms of right vs left). That's about when the switch between the parties had been completed, with the increasingly liberal Democrats losing so much of the (white) South. Oregon's votes have long been dominated by the socio-politically liberal Willamette Valley, where such a huge percentage of the population has always lived.

That's the key to those secessionist counties, too: they're politically conservative, but believe (correctly) that they have little political power in Oregon. That's because they're very sparsely populated. That's actually how democracy works, at least in theory: one person, one vote...and there aren't a lot of people/votes on far Eastern Oregon. I can certainly see those folks feeling more at home as part of Idaho, particularly if Cascadian secession efforts ever come to anything.