It can't hold as many screaming fans as the football stadium in Denver, where Sen. Barack Obama is to address an expected crowd of 75,000 on Aug. 28. Nor is it conveniently located near the scenic Mississippi River, like the arena in St. Paul, Minn., where Sen. John McCain is to take the stage a few days later. But the 3,000-seat auditorium at pastor Rick Warren's Saddleback Church in Lake Forest, Calif., does have one notable advantage: It's a stage that Obama and McCain recently agreed to share, at least for a little while, on Aug. 16.

Saddleback is a "megachurch," a term used to describe Protestant Christian churches that have an average weekly attendance of 2,000 people or more. The roughly 1,300 U.S. megachurches attract more than 4.5 million worshippers each week, and perhaps twice that many people identify themselves as members. Megachurches represent roughly one half of 1 percent of the total number of U.S. congregations, but these massive, highly influential places of worship attract inordinate attention — which may be why McCain and Obama agreed to what Warren is calling a "civil forum."

And for both the Democratic and the Republican candidate, things will probably stay civil. Many people think of megachurches as far-right, evangelical, politically active strongholds. But Saddleback, the fourth-largest church in the country, could be fairly neutral territory.

Research we conducted in 2005 found that only 16 percent of megachurches had engaged in organized political involvement over the previous five years. Three-quarters of megachurches said they had never done so. Our current research suggests the same pattern: No more than a quarter of these churches engage in any overt political activity.

At the same time, however, many megachurch pastors are consummate politicians, keenly aware of the power of their platforms. I've seen numerous megachurch worship services at which politicians vying for positions from local school-board seats to the presidency have greeted the congregation, posed for pictures with the pastor and shaken hands with parishioners after the service. I've also seen megachurch pastors skillfully persuade town, county and state officials to work on behalf of a church's future, based on the votes the churchgoers could deliver.

Most of these pastors are savvy enough to straddle the fence on controversial issues such as our presence in Iraq or universal health care. They are mindful of their congregants' divided views on social and political issues such as whether women should work outside the home and how far to extend the rights of gays. Half of all megachurches describe their attendees as "somewhat on the conservative side" politically. Earlier, we found that while these congregations are overwhelmingly Republican, one in five parishioners was likely to be a registered independent or Democrat. So neither McCain nor Obama will be preaching directly to the choir.

Scott Thumma is co-author of "Beyond Megachurch Myths" and a professor at the Hartford Institute for Religion Research