John McCain and Barack Obama risk being seen out of their element and off their talking points Saturday night when they join influential pastor Rick Warren for a forum at his California mega-church.

But the unusual event also will provide an opportunity for the political rivals to woo conservative Christian voters, a bloc that has largely supported the Republican Party but that Obama nevertheless has courted.

The importance of the evangelical vote is only underscored by Obama's and McCain's willingness to join Warren at his Saddleback Church for the forum. It is their first such appearance together since they clinched their respective nominations. Obama has resisted calls from McCain to join him for a set of town hall meetings.

"The fact that this event is even taking place is significant," Republican strategist Doug Heye said. "It could provide an opportunity for Barack Obama to make some inroads, but I think you'll see John McCain stressing his conservative credentials."

Watch the forum live on FOX News starting at 8 p.m. ET Saturday.

Obama's appearance at the forum will underscore the gulf between him and America's most conservative religious voters on issues such as abortion and gay rights. If those issues come up, it will give McCain an opening to align himself with mainstream evangelicals.

Conservative Christians compose about one-quarter of the U.S. electorate.

But McCain is seen as uncomfortable talking about his personal beliefs and has not created much excitement among the bloc that helped elect President Bush. Many evangelical leaders have backed McCain only reluctantly. And he put conservative Christians on edge this week by floating the prospect of picking a running mate who supports abortion rights.

Rather, it is Obama who has actively sought religious voters.

"Usually they go Republican. The fact that Obama is in there with a chance of maybe getting a share of that vote is in some respects a great triumph for him," Democratic strategist Susan Estrich said. "To be perfectly blunt it's hard to imagine Al Gore or John Kerry doing very well at this kind of a forum, but Barack Obama speaks very movingly ... about issues of faith and values."

Following Obama's lead, the Democratic National Convention Committee announced Saturday that the upcoming Denver convention will include the first-ever "faith caucus meetings," in which faith leaders will host panel discussions on faith and moral issues.

The Saturday forum with Warren, best-selling author of "The Purpose-Driven Life," is expected to stray from traditional talking points and hot-button issues. The influential pastor will speak to each candidate separately for just under an hour.

Obama will go first, and McCain will not be allowed to hear the questions that are being asked. The two candidates then will meet briefly on stage for a handshake and photo opportunity before McCain takes his turn.

It will be a rare opportunity to see the candidates side-by-side, before they head into their respective national conventions over the next few weeks. Their contrasts, both physical and stylistic, will be on display in a way that the candidates' rapid-fire ads do not capture.

Warren himself is at the forefront of a movement among conservative Christians to broaden their public affairs agenda beyond litmus-test issues like abortion and gay marriage.

He is a leading advocate for HIV/AIDS victims worldwide. His PEACE Plan sends church members on humanitarian missions around the globe, where they address challenges like poverty, disease and illiteracy.

Warren representative A. Larry Ross said the forum Saturday night will cycle through four subject areas: stewardship, leadership, worldview and America's role in the world.

Warren, though, is running against the grain of some Christian conservatives who want to keep focus this election year on issues like abortion.

On the same day as the Saddleback forum, thousands of evangelicals gathered on the National Mall in Washington, D.C., where leaders like former presidential candidate Mike Huckabee called on Christians to stand strong on key moral issues.

The Associated Press contributed to this report.