Gerrymandering makes districts extremely sensitive to population changes. The country was heavily gerrymandered by Democrats until the 1990s, and Republicans managed to flip it around within the decade: https://upload.wikimedia.org/wikipedia/commons/thumb/e/e6/Po...

The chart shows the House popular vote compared to the number of House seats. When Biden entered Congress, Democrats were winning 10 percentage points more of House seats than their share of the popular vote. But those districts were extremely sensitive to fluctuations and after redistricting based on the 1990 census many flipped rapidly.

The Republican party was the party of the "wealthy elite" back then. They rebranded themselves as the standard bearer of Christian values to boost their voter base. Now their base is so hooked on this tribal identity they will vote for candidates whose behavior is antithetical to the stated party platform.

The Republican Party has been a fusion of business elite and evangelical Christians since its inception. The two groups not only broadly overlapped, see https://en.wikipedia.org/wiki/Quakers, but both had adverse interests to the slaveholding south. The religious segment opposed slavery, and the business segment favored economic policies favorable to manufacturing industry (e.g. a strong currency, tariffs, and infrastructure) versus the weak currency and low tariffs favored by the agricultural south.

The strength of each faction waxes and wanes depending on what’s dividing the electorate. During the mid-20th century, both parties embraced “Christian values” (https://content.gallup.com/origin/gallupinc/GallupSpaces/Pro...) so economic issues dominated. Since Clinton, the parties are generally both on board with neo-liberalism, so religious and social issues dominate.

I really don't think that one can draw a line from the Quakers to the modern evangelical Christians. The Quakers were/are probably the closest Christian denomination to the actual lived beliefs expressed in the New Testament.

Maybe I just miss this, but that doesn't seem to match up with current evangelical support of the Republican party.

You can't gerrymander the senate though, and the 2020 election was 53-44.