Thomas Hart Benton

If this immigration made Kentucky and Tennessee, and afterwards Missouri, less distinctively Southern in character than the South Atlantic States, it at the same time, by furnishing the first and for some time the most numerous element in the population of the states north of the Ohio, made the latter less characteristically Northern than was the case with those lying east of them. Up to 1810 Indiana kept petitioning Congress to allow slavery within her borders; Illinois, in the early days, felt as hostile towards Massachusetts as did Missouri. Moreover, at first the Southern States west of the mountains greatly outweighed the Northern, both in numbers and importance.
Thus several things came about.