a shift of 240k people from oregon to idaho would result in oregon going back down to 5 congressional districts, and idaho gaining one for three. so one electoral vote moves from a reliably-democratic state to a republican one. that one elector could very well swing a presidential election.

iirc, changes to state boundaries requires approval of both states and congress (and also the president, who would have to sign-off on the legislation passed there). oregon would never go for it--not entirely sure idaho would be on-board, either, even with the thought of gaining a congressional seat. providing state services and funding to that region would be a perpetual net-drain on idaho's economy.