Idaho seats legislators based on Oregon certified elections/processes? Essentially those same legislators would be seated in both Oregon and Idaho? Certainly both states must have rules prohibiting that.
In the end, what would be the value to Idaho of an Oregon state legislator sitting in their body? They get no tax revenue, no control over laws. Recognizing we are talking an impossible hypothetical, I don't see any benefits to Idaho pretending that these Counties are part of their state.
That isn't how it would work. Obviously, I'd bet on one of us getting struck by lightning twice in the same day before I'd bet on this happening, but here's how it would work. Oregon and Idaho agree for Oregon's eastern counties to be annexed by Idaho. Everything that is decided by state law which is a lot (state taxes, elections, education, public safety, and public health) would be transfered to Idaho. It's not a unilateral secession. It's an agreed shifting of the borders secured by an interstate compact.
Like I said above, the federal issues are where it would get dicey. The new citizens of Idaho would be represented in Congress in districts drawn by Oregon officials, and of course, the allocation of federal money would be a mess. (That kind of thing is complicated as it is.)
Incidentally, WA has a hint of this same secession movement. Like OR, the Cascades dividing the state create a geographically driven cultural and economic divide. 70% of the population and >GDP sit in the Western parts of the States.
Similar splits exist in nearly every state. @OUBubba called it when he joked about Austin seceding from TX. Simply put, secession from a State is not a viable relief valve for resolving political differences. What changed is that the extremes are less willing to compromise. Instead, compromise is considered weak because the opponent is "evil". Any shift in their direction is a loss, a slippery slope towards the end. We all need to stop embracing the extremists.
I agree for sure in the short term. I don't think it would make a big difference in Idaho, because it's already a conservative state. However, there'd be no one to say "no" or even "slow down" in Oregon. They would go hard Left. However, in the long term, I'm less certain. The Left would wildly overshoot. However, the Oregon GOP would no longer be pulled by the rural wing and would likely moderate back into being a center-Right/keep taxes low/make the trains run on time suburban party again. That would likely make it competitive again in Oregon. It wouldn't take over the state, but it might be able to compete well enough to pull Democrats from the far left.