I read an article about this recently, written by an evangelical Christian trying to understand the evolution of evangelicalism and its concomitant religious movements in the United States. The article is framed around Donald Trump, but he isn't its sole focus: it's a broader analysis of how American evangelicalism developed its intensely conservative-political tinge.

"The Last Temptation," by Michael Gerson, The Atlantic April 2018

A sample:

Some words, like strategic castles, are worth defending, and evangelical is among them. While the term is notoriously difficult to define, it certainly encompasses a “born-again” religious experience, a commitment to the authority of the Bible, and an emphasis on the redemptive power of Jesus Christ.

...The corruption of a political party is regrettable. The corruption of a religious tradition by politics is tragic, shaming those who participate in it.

How did something so important and admirable become so disgraced? For many people, including myself, this question involves both intellectual analysis and personal angst. The answer extends back some 150 years, and involves cultural and political shifts that long pre-date Donald Trump. It is the story of how an influential and culturally confident religious movement became a marginalized and anxious minority seeking political protection under the wing of a man such as Trump, the least traditionally Christian figure—in temperament, behavior, and evident belief—to assume the presidency in living memory.