Until the 20th century, the difference wouldn't be that major; apart from Virginia and New York, no state had serious claims to the midwest (and no, Cleveland doesn't count as serious; although CT could always have tried selling it to PA to be assholes). The over population of the eastern half of the united states in 1901 was 57 million (vs 19 for the rest). While immigration would probably lower that amount, there's relatively good chances that the north American republics may still have a very very large part of the american population, although certainly not the overwhelming 45% that the United States had in 1901. I would expect the disunited states to consolidate themselves in 7 or 8 countries (with New England banding together, the deep south banding together, Delaware and PA reuniting, leaving us Virginia, Maryland, New Jersey and New York, plus maybe Vermont which did fight against New York for independence, so if it joins anything, it's as a NE state)
I also suspect that the 48-ers who moved from Germany (and the rest of Europe to a lesser extent) would be more spread out; a number of them did get to Mexico IOTL after all, and without the Mexican American war, there might not be such things as the Santa Anna presidency.
I'd thought at some point about making a reverse America thing, where instead of Brazil and the US holding together, the Spanish colonies somehow managed to hold as three or four countries and the portuguese and british colonies fell apart instead. I had a significant part of the great plains acting as the equivalent of OTL Bolivia and Paraguay (i.e. native majority nations)