This is sloppy and full of holes but it's the best I can make of a really difficult challenge. A Reconstruction plan that encourages (forces?) black migration to the interior leads to disfavored status for those states as a destination for white settlement. Fewer white immigrants chose to settle and those that do are of a more egalitarian bent. The industrial revolution happens apace, maybe with slightly different patterns of factory building to allow for the lower wages of the interior (but still trade route access is going to be the main factor). Red states are a mixture of egalitarian progressives and labor populists. Blue states are a mixture of ancient farmsteading conservatives and what you might call the country club set.
While the blue states still have red voters in the big cities (see Washington DC) suburbanization happened earlier and factory jobs moved to cheaper interior states earlier than the southern migration of factories seen IOTL. Similarly, blue pockets exist in the red states but many rural blues moved back to the south or to the west coast, and a lot of absentee management class types run things from the coasts.
This is a particularly partisan election. Normally purple/swing states include NY, NJ, MD, PA, OH, IN, MI, TN, AK, MO, OK and maybe the southwest depending on later patterns of settlement well past the POD.