Its white Christian nationalism, plain and simple. The alliance is born out of racist grievance politics.
The Bible Belt is a bunch of hypocrites. They don't give a sh**t about the morals, its about who is in control and gets to define American culture. And the north isn't becoming more religious, it's becoming more openly racist which has lead to a natural alliance with white people in the south who have mostly never stopped being openly racist.
This is a post that barely has anything to do with the topic at hand and little more than a rant to confirm your ideological priors. It's laughable, for instance, to say the North is more openly racist than it was 30 years ago.
I'm curious as to where you live because this has definitely been my experience as a person who has lived in Ohio my entire life. I've seen multiple family members flip this switch since the "birther" movement began. People are turning off filters that they used to employ to hide or tamp down their racism because Trump made it OK to be publicly racist.
And my post is very much on topic. OP literally asked how to reconcile the Rust Belt with the Bible Belt and the answer is, as I said, white Christian nationalism and racist grievance politics. A big part of understanding that is the acknowledgement that the Bible Belt cares more about maintaining political power than it does about actual Biblical values. If Evangelical voters actually cared about Biblical values they wouldn't continue electing morally bankrupt leaders like Trump. What they really care about is maintaining power so they elect whatever strongmen promise to help them maintain that power. The South has been that way since Reconstruction.
Generally speaking, people in the North/Rust Belt have always tried to be less direct with their racism because they don't actually believe they are racist in the first place. These are the people who used to start sentences with "I'm not a racist but ..." or "I have a black co-worker who is great but ..." People were afraid of being publicly labeled as racists so they tried to conceal it. But when Trump came along he essentially ended the this sort of "polite racism" and now those same people will just flat out tell you that blacks, Muslims, China, immigrants, etc. are ruining the country for hard working whites like them. They talk about things like "reverse racism" and try to claim that white people have it way worse than minorities because diversity efforts are causing more qualified white people to be rejected for jobs in favor of less qualified racial minorities.
