Considering Republicans crush it in Oklahoma, why are these counties (comparatively) good for Democrats? There doesn’t appear to be a big city here. There are high numbers of Natives here, but they are still majority white. Democratic Governor candidates have won these counties frequently and in the 2018 house races they both gave the Democrats over 40% of the vote.
Muskogee is the 6th largest city in OKlahoma, excluding suburbs after OKC, Tulsa, Lawton, Stillwater, and Enid, though its population has stagnated over the past 100 years.
So it has never attracted an entrepreneurial middle class that would vote Republican, but may have been large enough to maintain a party establishment. In a more rural county the party establishment would more likely be the community establishment and not tethered as strongly to a political party. Muskogee has an 18% black population, and the Indian population may form more of an underclass concentrated in an urban setting.