The event will play to one of Obama's strengths, talking about his Christian faith, but it will also underscore the gulf between his views and those of the most conservative Christian voters.
Many of McCain's positions are more in line with the evangelical worldview, but he is uncomfortable - and some critics say unconvincing - while talking about his personal beliefs.
The candidates will appear separately, spending one hour each with Warren, before coming together on stage for a handshake. The pastor, who does not endorse candidates, will be the only one asking questions.
Warren is an anti-abortion Southern Baptist who is nonetheless part of a shift away from the religious right's strict focus on abortion and marriage. The environment, poverty and education have also become pressing concerns, especially for younger evangelicals.
Warren is best known for building Saddleback Church into a 23,000-member megachurch in Lake Forest, Calif., and for writing the multimillion-selling book "The Purpose-Driven Life."
But he and his wife, Kay, are also leading advocates for HIV/AIDS victims worldwide. They have invested enormous resources in their PEACE Plan, now under way in Rwanda, which aims to combat corruption, illiteracy and other social problems through church partnerships with government and business.
Older-guard evangelical leaders who oppose broadening the agenda have been leaning on Warren. In a stream of statements in the days leading up to the forum, they implored him to press the candidates about their positions on abortion.
Larry Ross, who represents Warren, said the pastor has been consulting with other clergy and with experts in different fields to develop questions for the candidates about leadership, the Constitution, human rights and "sin and righteousness issues."
"The more liberal camp just assumes that Pastor Warren is going to make this a Christian litmus test of the presidency. Others, who are more conservative, fear he is going to wimp out on some of the issues," Ross said. "He says, 'Neither group understands or knows me.' He's going to ask tough questions, fair questions, not gotcha questions."
Obama has proven adept at explaining how his Christian faith has shaped his policies. The church forum also gives him a perfect setting to counter the misperception that he is Muslim. A recent poll by the Pew Research Center for the People and the Press found that 12 percent of respondents believe the Illinois senator is Muslim.
"It's a great way for him to do what he can to make connections with not only moderate evangelicals, but also the many people out there who read 'The Purpose-Driven Life,"' said Mark Silk, who specializes in religion and public life at Trinity College in Hartford, Conn.
However, Obama will inevitably be asked to explain his support for abortion rights and other issues that clash with conservative Christian theology.
The Obama campaign has been diligently courting religious voters with a presence on Christian radio and blogs, and through "American Values Forums" and other events.
In June, Obama took the bold step of holding a private meeting with a large group of evangelical leaders, including the Rev.
Franklin Graham, who challenged him on his beliefs in salvation, his support for abortion rights and other issues.
The benefit of the forum to McCain, who attends a Baptist church, is less clear.
While many of his views, including opposition to abortion, match the outlook of conservative Christians, he is far less comfortable than Obama discussing his faith. McCain did not participate in a spring forum at Messiah College near Harrisburg, Pa., where Obama and New York Sen. Hillary Rodham Clinton discussed religion and their personal lives.
McCain supporters have taken to circulating excerpts from his memoir "Faith of Our Fathers," that explain his beliefs. He recently met privately with Roman Catholic Archbishop Charles Chaput of Denver, one of the most vocal U.S. bishops on the duty of Catholics to make the abortion issue a priority in choosing public leaders.
Yet, many evangelical leaders have backed him only reluctantly.
And he put conservative Christians on edge Thursday by floating the prospect of picking a running mate who supports abortion rights.
Conservative Christians comprise about one-quarter of the electorate.
"You just wonder, is he trying to shoot himself in the foot?"
said David Domke, author of "The God Strategy: How Religion Became a Political Weapon in America."
No one expects Obama to lure the most traditional Christian voters from the GOP. Polls consistently show McCain winning frequent churchgoers by large margins. But in a close general election, Obama could win by taking a small percentage of the evangelical vote away from the GOP.
"Obama is going to make real inroads for people who want to be satisfied that this is a pretty religious guy but that he's not a lunatic," Silk said.
The person with the most at stake may be Warren himself. The impression he makes Saturday will shape his reputation, the public view of his church and his position among evangelicals for a long time to come.
"I think Rick is in an unenviable position in that he stands to get attacked from the right and the left, based on what direction he takes," said Mark DeMoss, an evangelical public relations specialist who had supported former Massachusetts Gov. Mitt Romney in the GOP primary. "As an evangelical, I am much more interested in his list of questions than in either of their answers."---
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