Businessman David Koch arrives at the Metropolitan Museum of Art Costume Institute Gala 2015 celebrating the opening of "China: Through the Looking Glass" in New York, May 4, 2015. Reuters

The author of a book on the influence of conservative donors on American campaigns decided not to focus on the general effect of money on politics but rather two of the major factors: looking deeply into Charles and David Koch, two Midwestern oil company owners who have since the 1980s spent millions to sway presidential and congressional elections.

The book, “Dark Money: The Hidden History of the Billionaires Behind the Rise of the Radical Right,” by Jane Mayer of New Yorker magazine, documents the history of the brothers, whose Wichita, Kansas, company is the second-largest private company in the U.S., and their politics, the Economist reported. While they have viewed the ascendency of Barack Obama to the presidency as apocalyptic, they have spent much of their money on elections at the state level where most business regulations are concentrated.

While the Kochs’ lives have been under public scrutiny for years, Mayer’s book focuses on the brothers’ early lives and the origins of the company — alleging the family amassed at least part of their fortune by building oil refineries in Nazi Germany — to give context to the Kochs and their influence, the New York Times reported. Charles and David Koch tried to blackmail another of their brothers, Frederick, to sell his shares of the company by threatening to tell their father he was gay, something Frederick has denied.

Mayer alleges in the book the Koch brothers and others like them have created a “political bank” that allows them to dole out enormous sums of cash to their favorite candidates without having to disclose, in large part, the origins of the money. They use the Internal Revenue Code, which allows them to throw money into the political ring essentially unregulated.

Charles and David Koch didn’t want to talk to Mayer for the book but are set this weekend to convene a retreat with hundreds of other wealthy conservative donors in California, the Guardian reported. The retreat, and the release of Mayer’s book, come at a time when it is expected conservative donors will spend about $900 million to influence this year's political campaigns.