When it comes to a non-peer-reviewed or non-scholarly-theory based definition of a primary tension in U.S. Evangelicalism I don’t think you can do much better than how @skye.jethani frames it:

{quoting a Christian bookstore owner}: “It’s our job to find out what our customers need, and then seek to meet those needs. Any Christian bookstore that sets out to change the customer’s desires is headed for bankruptcy.” And I think this is what they both get right about the American white evangelical experience, and that is it is largely shaped by the marketplace. In that it’s a populist movement that if you go against popular desire, you’re going to lose audience, you’re going to lose donors, you’re going to lose customers. And so it responds to what the market wants. And I think the way Bonnie {Kristian} deals with that is she looks at what has been insurgent in American popular culture in the last ten years, and a lot of it’s ugly stuff, and she said, “When that comes into American evangelicalism, that’s the result of American culture. It’s not evangelicalism.” And so don’t blame evangelicalism for it. And I think Kristin {Kobes du Mez} looks at that and goes, “Yeah, but evangelicalism, because it’s a populist movement, lacks the strength to disciple these bad things out of its people.”