Evangelical Christianity is a broad revival tent, sheltering about a quarter of America’s population — from mainline Protestants to Roman Catholics, with a long list of other labels in between. Since the nation’s founding, evangelicals have played influential roles in shaping culture, and lately politics.

Frances FitzGerald, a writer whose reach is almost as wide — from the Vietnam War (“Fire in the Lake”) to President Reagan’s Star Wars program (“Way Out There in the Blue”) — ambitiously tries to capture this essential part of American history, but the essence of evangelicalism eluded her. Even though she limited her subject to white people, the topic appears too complex and fragmented to encompass all its parts.

“Evangelicals” is a sincere work that starts with a definition: Evangelicals see themselves “born again” in Jesus Christ and having a duty to spread the “good news” of the gospel around the world.

FitzGerald says that definition is little changed from the 19th century, when a series of “great awakenings” or revivals erupted around the nation. “For most of the 19th century almost all Protestants would have called themselves evangelicals,” she writes.

Her history of that time, however, is so full of details that it seems aimed at a religious history student rather than the general reader. Her list of figures in the movement reads almost like an Old Testament genealogy of “begats.”

When FitzGerald finally reaches the 20th century, the shape of modern evangelicalism is in place, but in essentially one place, the South. It set itself apart from the country, first in its espousal of slavery, then after the Civil War, in segregation. In the present century, she writes, conservative and moderate evangelicals make up a large part of the Republican Party, are a majority of the so-called “Tea Party” movement and support politicians who oppose same-sex marriage, the Affordable Care Act, immigration reform, programs for the poor, access to abortion and curbs on religious expression. They tend to be white, middle-aged men with a comfortable income, many of whom live in the South.

FitzGerald delves into the various groups known as the “Christian Right,” including Focus on the Family, which flourished in the 1990s, fueled by disgust with President Bill Clinton. She chronicles what she sees as the decline of that influence, quoting the moderate evangelical the Rev. Rick Warren, who declared the “Christian Right is dead” and who delivered the invocation at President Barack Obama’s first inaugural.

FitzGerald points out that Obama worked hard to include evangelicals in his 2008 campaign and sought compromises with religious groups, including Catholics, to pass the health care bill. Previously, Democrats had ceded evangelicals to the Republicans, but the Obama campaign recognized the need to reach out to these groups, FitzGerald says.

President Donald Trump, despite his behavior, drew strong support from many evangelicals. FitzGerald points out that those voters identified with the attitudes of the Tea Party from a nostalgia for an earlier largely white America to anti-immigrant policies.

“Evangelicals” might be seen as a work in progress as the population of white Protestants — the traditional element of the movement — declines and is replaced by a growing number of nonwhite citizens. Along with its history, FitzGerald might have written its elegy.

Bob Hoover is the retired book editor of the Pittsburgh Post-Gazette.

The Evangelicals: The Struggle to Shape America
By: Frances FitzGerald.
Publisher: Simon & Schuster, 740 pages, $35.