I can only imagine this sort of thing happening if blacks were bound to the land and kept from migrating from the South while whites were free. I don't know what would allow this: some sort of debt peonage enforced by states? continued slavery of some kind?
It is actually highly plausible to imagine a much larger white migration from the South. Ever since the beginning of westward expansion, free states had wanted to exclude free blacks to avoid them competing for jobs with whites. In fact, with the expansion of the US, racial prejudice quite clearly
increased: whereas a number of the original states allowed free people of colour to vote under certain conditions (the majority removed this right before the Civil War) none of the states admitted after 1800 ever did.
All that would be required for a much larger white migration from the South is for free states to become more rigorous in their exclusion of free blacks after the 1850s, which is a highly plausible result if there had been no Civil War: even with the Fifteenth Amendment many local governments completely excluded blacks from residing locally as late as 1970. Thus, we could see the West, and increasingly other states, excluding free blacks and attempting to attract other migrants. With free states (possibly excluding the Northeast) excluding black labour almost completely, Southern poor whites would become the logical (
only) choice once European conflicts rendered that continent less able to supply labour to the North and West.
Under this scenario, if we assume European conflicts are not affected, the US becomes controlled outside the old northeastern and Great Lakes states by a Democratic Party that managed to escape the Civil War and the conversion of mountaineers to Unionist Republicanism. The Southwest becomes admitted as slave states to balance the Plains as free states, and with the growth of irrigation black slaves become a key labour force in the Colorado and Rio Grande valleys on irrigated cotton plantations. The free states do keep slavery out of the northwestern territories (an accidental “Second Missouri Compromise”) but abolitionist movements actually
weaken with time as “scientific racism” becomes more and more dominant at all levels of American society.
When World War I breaks out, the US is not keen on entering the war but does work to help Britain. The Republican Party takes over a large part of the Heartland and West in 1920 as a result of the failure of Democratic war policies, and aims to recapture economic growth and eliminate US involvement in foreign class struggles after the war. At the same time, exhaustion of poor upcountry soils leads to abandonment of dirt farms in the area and the expansion of still-slave plantations in the South, whilst political turmoil leads the Republicans to effectively close the country off to all but Northwestern European migrants. Consequently, the Upland South loses a large proportion of its poor whites to industrial cities of the Great Lakes and the West, who leave the Deep South with an ever-increasing majority of slaves, even as Maryland and Delaware had abolished slavery in the 1890s and Kentucky began a plan for this in 1905. A rigid fugitive slave law and continuing racial hostility means that it is impossible for blacks to move away from the South, but the poor white land become owned by rich plantation owners, whose only request from the Federal Government is research into and control of the boll weevil in the Southeast.