Dumping a bunch of people sucking down more money than they pay seems like a win for Oregon.
Idaho, on the other hand, might object. That would be comedy gold.
"We secede." "Congratulations! Good luck finding somebody who wants you, though."
Do you mean the House? I think the Senate would still have 2 senators per state, and state-level electoral results are unlikely to change if an alienated minority from one state secedes, in favor of joining a more (politically) similar state.
For example if upstate NY joined Pennsylvania, then the D senator from PA would very likely lose. Or if (Republican-leaning) inland California joined Nevada, then Nevada would probably tip red. Or maybe (heavily blue) Jacksonville and Gainesville get fed up with DeSantis, and leave Florida for Georgia, cementing GA as a solid blue state.
While your question is still an interesting thought experiment, it is worth noting that OP was referring to counties moving between two existing states, not counties seceding a former a _new_ state, which would certainly have different ramifications.
And are counties only allowed to do it if they are contiguous with their desired state?
The more worrisome effect is on the electoral college and the senate, but I think tying this to DC statehood would get rid of that.
The [Republican] Idaho legislature would get to gerrymand the districts instead of the [Democratic] Oregon legislature.