Which states do you suspect will never vote Democratic or Republican again....

[Blackacre]

DC will never vote Republican

Oklahoma will never vote Democratic.

That's all I got.

Wyoming is an interesting case. It, the Dakotas, Montana, and Idaho all have very Republican populations, but also very small populations. Small enough that any of those states could get the Colorado or Vermont treatment from a large enough population shift. (of the five, Idaho is the most insulated, but not by much)

Oklahoma is reasonably large (comparable size to Connecticut. Not a big state, but not tiny either) and uniformly white, rural, and conservative. This means it is more immune to interstate population shifts than Wyoming and also would never vote for a Democratic Party that even remotely resembles the one that currently exists. It's not Appalachia, nor is it mormon. So Oklahoma is my answer.

If I had to make an additional answer for each party, I'd say California for the Republicans and Arkansas for the Democrats. Cali is huge, diverse, liberal, contains expensive media markets, and is used as a foil for cultural conservatives. Arkansas is not Appalachian nor does it contain a high black population. So of all the heavily GOP Southern states, I'd rate Arkansas as the least likely to support the Democratic Party in my lifetime.