When John McCain and Barack Obama appear on the same stage Saturday at the sprawling religious campus of Orange County’s Saddleback Church, their presence will vividly underline the reach that has made Pastor Rick Warren among the most significant evangelists of his generation.

But the joint appearance — one of Warren’s highest-profile endeavors — will also underscore a tension that is central to his role.

Warren has been called perhaps “America’s most influential pastor,” an evangelical megastar who leads the nation’s fourth-largest church, reaches thousands of ministers through the Internet and crusades against poverty and AIDS.

That globe-trotting work — and his phenomenally successful book, “The Purpose Driven Life” — have propelled him into the vanguard of a movement that inspires young and socially conscious Christians.

But Warren’s willingness to soft-pedal political issues once central to U.S. evangelicals, such as opposition to abortion, has opened him to criticism that he has strayed from his calling to spread the Gospel.

It’s likely that both fans and critics will be watching closely when Warren plays host to the two presidential contenders at his church complex in Lake Forest, home to 22,000 weekend worshipers.

The presumptive Democratic and Republican nominees won’t debate during the Civil Forum on the Presidency. But they will make a brief joint appearance, their first of the campaign, and Warren will interview each separately about the Constitution, poverty, AIDS, human rights and other subjects.

“America has a choice. It’s not between a stud and a dud this year,” Warren said. “Both of these men care about America. My job is to let them share their views.”

Many evangelicals believe that Warren’s growing profile, and his willingness to welcome Obama to his pulpit, are evidence that he has emerged as the most pivotal figure in U.S. evangelicalism.

The 54-year-old pastor, they say, is emblematic of a new breed of evangelicals who put social justice ahead of partisan politics. Some go so far as to call the plain-talking Warren, a bear of a man who prefers bluejeans to business suits, the Billy Graham of his era.

“He’s a guy whose message has met the right moment,” said Richard Land, a leading authority with the Southern Baptist Convention, the denomination to which Warren’s church belongs.

An excerpt from a letter Warren sent to his congregants suggests his reach. He noted that three Republican and three Democratic presidential candidates contacted the church during the primaries:

“You know that I never endorse, nor campaign for, political candidates. Neither is it my role to give political advice. But I am a cultural observer and I do understand the unique stresses and responsibilities of public leadership, so I try to help leaders when asked.”

But detractors see Warren as a spiritual entrepreneur who has built his religious empire on what they call generic self-help ideas found in “The Purpose Driven Life.”

“For many evangelical leaders, Rick Warren is either a little too naive or a little too shrewd,” said the Rev. Rob Schenck, president of the National Clergy Council, a Washington group that works to meld Christian teachings into the debate over public policies.

“He is threatening to water down the essential message of evangelical Christianity,” Schenck said. “And that is what causes people to grow a little insecure and concerned, and maybe even disconcerted.”

Warren insists that he remains firmly tied to his Southern Baptist roots.

He opposes abortion and defines marriage as the union of a man and a woman. He has hosted politically conservative figures, such as Sen. Sam Brownback (R-Kan.).

But Warren says he is also inspired by the broader message of faith and compassion in the Bible.

The forum with McCain and Obama, he said, is his latest attempt to introduce civility into public discourse, even if it irks some of his fellow evangelicals. Warren faced biting criticism in 2006 when Obama spoke at his church for a global AIDS summit. Last year Sen. Hillary Rodham Clinton (D-N.Y.) appeared at another AIDS conference at the church.