Today the movement shows signs of coming apart beneath its leaders. It is not merely that none of the 2008 Republican front-runners come close to measuring up to President Bush in the eyes of the evangelical faithful, although it would be hard to find a cast of characters more ill fit for those shoes: a lapsed-Catholic big-city mayor; a Massachusetts Mormon; a church-skipping Hollywood character actor; and a political renegade known for crossing swords with the Rev. Pat Robertson and the Rev. Jerry Falwell. Nor is the problem simply that the Democratic presidential front-runners — Senator Hillary Rodham Clinton, Senator Barack Obama and former Senator John Edwards — sound like a bunch of tent-revival Bible thumpers compared with the Republicans.

The 2008 election is just the latest stress on a system of fault lines that go much deeper. The phenomenon of theologically conservative Christians plunging into political activism on the right is, historically speaking, something of an anomaly. Most evangelicals shrugged off abortion as a Catholic issue until after the 1973 Roe v. Wade decision. But in the wake of the ban on public-school prayer, the sexual revolution and the exodus to the suburbs that filled the new megachurches, protecting the unborn became the rallying cry of a new movement to uphold the traditional family. Now another confluence of factors is threatening to tear the movement apart. The extraordinary evangelical love affair with Bush has ended, for many, in heartbreak over the Iraq war and what they see as his meager domestic accomplishments. That disappointment, in turn, has sharpened latent divisions within the evangelical world — over the evangelical alliance with the Republican Party, among approaches to ministry and theology, and between the generations.

The founding generation of leaders like Falwell and Dobson, who first guided evangelicals into Republican politics 30 years ago, is passing from the scene. Falwell died in the spring. Paul Weyrich, 65, the indefatigable organizer who helped build Falwell’s Moral Majority and much of the rest of the movement, is confined to a wheelchair after losing his legs because of complications from a fall. Dobson, who is 71 and still vigorous, is already planning for a succession at Focus on the Family; it is expected to tack toward the less political family advice that is its bread and butter.

The engineers of the momentous 1980s takeover that expunged political and theological moderates from the Southern Baptist Convention are retiring or dying off, too. And in September, when I called a spokesman for the ailing Presbyterian televangelist D. James Kennedy, another pillar of the Christian conservative movement, I learned that Kennedy had “gone home to the Lord” at 2 a.m. that morning.

Meanwhile, a younger generation of evangelical pastors — including the widely emulated preachers Rick Warren and Bill Hybels — are pushing the movement and its theology in new directions. There are many related ways to characterize the split: a push to better this world as well as save eternal souls; a focus on the spiritual growth that follows conversion rather than the yes-or-no moment of salvation; a renewed attention to Jesus’ teachings about social justice as well as about personal or sexual morality. However conceived, though, the result is a new interest in public policies that address problems of peace, health and poverty — problems, unlike abortion and same-sex marriage, where left and right compete to present the best answers.

The backlash on the right against Bush and the war has emboldened some previously circumspect evangelical leaders to criticize the leadership of the Christian conservative political movement. “The quickness to arms, the quickness to invade, I think that caused a kind of desertion of what has been known as the Christian right,” Hybels, whose Willow Creek Association now includes 12,000 churches, told me over the summer. “People who might be called progressive evangelicals or centrist evangelicals are one stirring away from a real awakening.”

The generational and theological shifts in the evangelical world are turning the next election into a credibility test for the conservative Christian establishment. The current Republican front-runner in national polls, Rudolph W. Giuliani, could hardly be less like their kind of guy: twice divorced, thrice married, estranged from his children and church and a supporter of legalized abortion and gay rights. Alarmed at the continued strength of his candidacy, Dobson and a group of about 50 evangelical Christians leaders agreed last month to back a third party if Giuliani becomes the Republican nominee. But polls show that Giuliani is the most popular candidate among white evangelical voters. He has the support, so far, of a plurality if not a majority of conservative Christians. If Giuliani captures the nomination despite the threat of an evangelical revolt, it will be a long time before Republican strategists pay attention to the demands of conservative Christian leaders again. And if the Democrats capitalize on the current demoralization to capture a larger share of evangelical votes, the credibility damage could be just as severe.