I think that the GOP has more or less maxed-out their support in most rural areas. I mean, they're already getting over 90% of the vote in some rural counties in the Mountain west and Great Plains states.
Definitely true, but there are still gains to be made in places like the NE and the Iron Range.
These areas moved towards the GOP in 2016 because Trump was perceived as being both secular and an economic populist, whereas voters in these regions had previously perceived the GOP to be the party of the rich or the religious right (or both).
I think the association with the religious right was the primary negative factor in the socially liberal NE, whereas the perception of the GOP as being the party of the wealthy was the more important negative factor in the Iron Range (and all across the Rust Belt)
In the medium-long term, I think the GOP can begin to make gains in one of these areas but not the other. It depends, I think, on whether the GOP trends more heavily towards secular cultural conservatism (fighting the liberal/media elites/SJWs, silent majority politics, but not necessarily tied to the religious right) or more heavily towards economic populism (mainly protectionism).
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