The NCR is a direct metaphor for Westward Expansion era America, complete with democratic ideals and a reality of rampant corruption, conquest of territory and the genocide of the natives, and a brewing slavery/“sharecropping” crisis. They even have a belief equivalent to Manifest Destiny, as they consider themselves the rightful inheritors of America, sea to shining sea.
The only question is whether that means the Legion is their metaphorical Confederacy, if their Slaveholder Rebellion is coming, or if maybe it’s silly to take the metaphor any deeper.