I disagree, so I think that's probably not the first thing to understand about US politics.
Oh, also, you're incorrect - according to Pew it's closer to 25%.
As of 2020, the number was 34% responding yes to "Would you describe yourself as a "born-again" or evangelical Christian?". In 2019, it was 37%, and 42% in 2017.
Pre-20th century Europe is not a great example of constructive political discourse btw: https://en.wikipedia.org/wiki/European_wars_of_religion
"born-again" and "evangelical" are not interchangeable terms, though there is significant cross-over. That's why you'll get a higher number if you ask if someone is either born again or evangelical than if you ask if they are evangelical alone.
The other stuff about this specific religion having 'irrational' beliefs I think extends to most religions generally so I don't think is that useful as a differentiator.
That isn't what the parent is arguing though.
Again, I don't think that the 'irrational beliefs' of evangelicals are unique to them, so as I said before if that's the point it also fails.
Breaking the argument down:
Statement: The first thing to understand about US politics is that almost 40% of the population is evangelical. There can be no constructive political discourse with that type of demographic skew.
So there can be no constructive political discourse because 40% of the population is evangelical. Why?
Reasoning: 1. In addition to the generally irrational beliefs held by evangelicals (e.g. the earth being 6000 years old), they 2. tend to vote as a unified block, and 3. The remaining 60% of the population generally has disparate interests and doesn't have a strongly unified vote.
As I said, throw out #1, because it isn't unique to their religion and you have two things: 1. an organized voting block with the sole unifier being religion and 2. a disparate remainder of the population with varied interests and religions.
The first part of the remainder is also bunk, because only 56% of evangelicals lean republican. 28% lean democrat, the remainder are unaffiliated, according to Pew. So even if you took the 40% total which isn't accurate, you'd have a voting block of only about 21%, with 10% going the other side.
But even if it were accurate, it again applies to any organized religion, unless you think that evangelicals are for some reason unique in their being aligned solely on the basis of religious belief. That would be a really weird argument given the wars fought over religion prior to the formation of protestant religions.
the second part, sure, the population has varied interests. No disagreement there.
You can see a breakdown here: https://www.pewresearch.org/fact-tank/2014/10/30/5-facts-abo...
The rest of your argument hinges on the rejection of evolution being shared among other groups and therefore being invalid, but as you can see, the rejection of evolution is largely a evangelical phenomenon.
In 19th century Europe and in modern central and south America, however, this is not associated with evangelical protestants, though protestants generally are more likely to have an issue with evolution.
Still, that's not really it - you've got a ton of irrational beliefs held by the catholic church. Unless you think those specific beliefs are what prevent constructive discourse?
They claim to be. But when some of them unironically rolled out a literal golden statue of their human leader, I saw no outrage from the rest of them. I saw no condemnation from the rest of them. I simply don't believe their claim.
But does it even matter whether we, you and I, believe them or not; isn't the main issue whether they themselves believe it? (And I'm afraid enough of them do.)