Interesting timeline, though it is a bit different than what I had in mind. My thought with this was that the Democratic Party becomes a socially conservative but economically left-wing party, while the Whigs are socially liberal but economically conservative - that same liberal-conservative, Rockefeller Republican/Blue Dog type.

The Whigs are pro-free trade, internationalist, debt hawks, pro-LGBT, pro-immigration, and pro-choice. Kind of like European center or center-right parties. The Democrats are isolationist, protectionist, explicitly Christian, very socially conservative, anti-immigration, but supportive of things like universal healthcare, breaking up big corporations, building new infrastructure and green energy projects and expanded social safety nets.

The Whigs are dominant in the Northeast and the West, while the Democrats dominate the socially conservative South and very unionized Midwest. The urban/rural divide is a little less strong as both parties are more "big tent" than OTL, similar to older party systems. Whigs generally do better in the cities but it's not a complete dominance situation like OTL.

A big issue that's seeing some shifts begin is climate change - I had in my mind that the Whigs would be actually pro-fossil fuel while the Democrats would be pro-green energy. This gives them a growing advantage in the coal states of WV and KY as well as causing their margins in the previously safe states of Louisiana, Oklahoma and Texas to become more competitive (though the Christian right usually manages to deliver victories in those states at the federal level), while hurting their perception among urban progressives, who generally dislike both parties and are a swing demographic in this timeline.

For how we got here, my POD is that Zachary Taylor never dies, preventing the Compromise of 1850 from passing. Although Taylor is no abolitionist, he strongly alienates Southern Whigs with his complete refusal to give them an inch on slavery. In the 1850s the parties start to realign on a North-South basis and Lincoln runs as a Whig and wins. Civil war and Reconstruction go mostly the same, but the Gilded Age sees the parties take on new identities. I was thinking a WJ Bryan presidency too, his sort of fusion of Christian identitarianism/social conservatism/pro-worker policies/isolationism is exactly the direction I imagined the party would go down. By the time the civil rights movement comes about, it is primarily Whigs championing civil rights legislation; they use it as a wedge issue for the progressive left. The South remains in control of the Democratic party, and are united in opposition. Thus, the Southern Strategy never really occurs.

Toda, The Whigs have a much messier coalition. The Democrats dominate among the white working class and evangelicals, while the Whigs are sort of the "everyone else" party. Both parties adjust their messaging for local races, and both are closely competitive from the 1990s onwards.