By Ed Stoddard

DALLAS (Reuters) - U.S. presidential rivals Barack Obama and John McCain target religious voters on Saturday when as guests of one of America's foremost evangelists they discuss faith in public life, AIDS, the environment and other issues.

Religion plays a big role in U.S. politics despite the traditional separation of church and state and the White House hopefuls are certain to be asked about how faith would fit in their potential presidencies.

The candidates won't debate each other at the Civil Forum which will be moderated by mega-pastor Rick Warren at his Saddleback Church in Lake Forest, California. He will interview each in turn, although they are expected to share the stage together briefly.

"It's quite an extraordinary thing, it's the first time a preacher has convened the two presumptive candidates ... They are both fighting for that vote," said Michael Lindsay, a political sociologist at Rice University in Houston.

Evangelicals account for one in four U.S. adults and have become a key conservative base for the Republican Party with a strong focus in the past on opposition to abortion and gay rights and the promotion of "traditional" family values.

Such issues delivered almost 80 percent of the white evangelical Protestant vote to President George W. Bush in 2004 but the movement is more fractured and restless this year though it remains largely in the Republican camp.

A survey in June by the Pew Forum on Religion & Public Life found that 61 percent of white evangelical Protestants supported the Republican McCain while only 25 percent backed the Democrat Obama.

But Pew noted that in June of 2004 Bush had the support of 69 percent of those surveyed from this group and other polls this year have shown growing pockets of white evangelical support for the Democratic Party.

Other surveys point to solid support for Obama and the Democrats from Hispanic and black evangelicals, making it a key "battleground faith" in the November 4 election.

MCCAIN AND ABORTION

McCain has not excited conservative evangelicals because of his past support for stem cell research, his blunt criticism of the movement's leaders in 2000 and other political heresies.

But the Vietnam veteran and former prisoner-of-war has long been opposed to abortion rights, a trump card with this group.

"McCain has a good record on that issue (abortion) and he must show that he will continue it as president," Tony Perkins, the president of the conservative lobby group the Family Research Council, told Reuters.

Analysts agreed that this was a big chance for McCain.

"For McCain the aim will be to solidify evangelicals as a key constituency," said Cal Jillson, a political scientist at Southern Methodist University in Dallas.

There is opportunity for Obama as well, a devout Christian who many observers say is far more comfortable and eloquent speaking about his faith than McCain, who grew up Episcopalian but who now attends an evangelical Southern Baptist church.

Many evangelical leaders including Warren have been pushing their movement to embrace a broader range of biblical concerns such as poverty and climate change, moving beyond though not excluding culture issues such as abortion.

Obama, who would be the country's first black president, has linked such issues pointedly to his faith.

"For Obama it is significant that he will be participating as an equal on the same stage as McCain in an evangelical church. This signals the shift in the evangelical political landscape since 2004," said David Gushee, a professor of Christian ethics at Mercer University in Atlanta.

Lindsay said while the setting is California, scene of a looming battle over gay marriage, the target would be politically undecided evangelicals in "swing states" where the White House race is forecast to be close.

"This has a lot less to do with what is going on in California and more to do with what is going on in Ohio, Pennsylvania and Florida, the big swing states," he said.

"In all of these states there is a sizable evangelical population that does not directly identify with the old 'Religious Right,"' said Lindsay.

The discussion will also no doubt be watched closely by Americans of other faiths such as Catholics, mainstream Protestants and Jews -- all voters whom both candidates will want to woo.

(Editing by David Wiessler)