That's the Mississippi River.
however, the mississippi river runs north and south and the blue line i am talking about runs east and west and does not appear to make it as far west as the mississippi. did you mean a tributary to the mississippi? others on this thread have mentioned it is because it is a fertile ground, perhaps containing the capital, that was once populated by large farms containing slaves. the hypothesis is that the once slaves are showing their preference for democratic policies.