Unrealistic. This falls at the first hurdle because while it is legally possible, those who advocate this don't understand the inevitable political circumstances and meta-context that would arise from this situation. This is nothing more than hypothetical and will never come close to reality.

The assumption that California and Oregon would be willing to give up land because their Democrat administrations hate Trump supporters is laughable. States are not ceremonial entities like administrative divisions in other countries, they have sovereign rights and ownership over land, even if the counties are sparsely-populated and vote 100% GOP then there will still be potential economic and strategic benefit for them to retain. States have historically gone to war over territory, they wouldn't give it up just because they don't like the people there. Why would Oregon give up 2/3 of its state and lose its direct land route to California just so it could be more be blue? Also, loss of land and population could also mean potential loss of electoral votes so why would Democratic states deliberately weaken their influence on the Federal government? This is why California partitioning plans fail as well, why would California deliberately weaken itself just to rid itself of Republicans who are already politically irrelevant.

Additionally, "Greater Idaho" would still be a subject under the US government and considering electoral winter has basically occurred prematurely then the Democratic government just has to impose federal laws on the state and use its federal powers any chance it gets.