The Rev. Rick Warren is so prominent and respected that just being seen with him is a boon for any presidential candidate.

For Republican John McCain and Democrat Barack Obama, their appearances at a forum Saturday night at Warren’s evangelical California megachurch bring risks along with rewards.

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.

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 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 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.

“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.