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.