I just don't see how that is possible considering the political leanings of the Dakotas at the time circa their admission. Seems to me that they were slightly more favorable to the Democrats.
Yes you are right and the GOP advantage was short lived as the Democrats began to win them. But in 1889 ND, SD, MT, ID, and WA were added as well as WY in 1890 to increase GOP power in the Senate and the electoral college. The GOP was considerable a Western Party by then. During the Speakership of Henderson 1899-1903 more then half of the GOP congressmen were from states that were considered Western at the time(Probably anything West of Ohio).