O.C. matchup between Barack Obama and John McCain is prelude to debates

But for the 3,000 people in the audience and viewers watching live on cable television, this first onstage matchup will offer a preview of the three critically important presidential debates, the first next month at the University of Mississippi.

Though appearing separately, the candidates will field similar questions about their faith, abortion, same-sex marriage and humanitarian efforts abroad. It is a chance for both to hone their comments on sensitive topics and practice connecting with an audience not chosen by their tightly controlled campaigns.

Debate analysts say that despite their different campaigning styles, neither McCain nor Obama will head into the debate phase with a clear advantage. "There's not such a great disparity in talent," said Northeastern University professor Alan Schroeder, author of "Presidential Debates: Fifty Years of High-Risk TV." "This isn't Bill Clinton versus Bob Dole."

Still, the campaigns have already begun trying to lower expectations. Obama spokesman Bill Burton noted that, with more than 20 primary debates, "we go into the fall, if nothing else, a tested organization." But he quickly added that McCain, after two decades in Washington, is "obviously a great debater." McCain strategist Charlie Black dryly noted that "in addition to being a great speechmaker," Obama "is pretty good on his feet."

In some respects, the candidates have opposite strengths.

Obama is often described as a powerful orator but was not a standout debater in the Democratic primary season. McCain is most comfortable speaking extemporaneously but has undercut his foreign-policy credentials with mistakes, such as mischaracterizing Iran's role in Iraq and referring to the Czech Republic as Czechoslovakia.

"This is like a high-stakes trapeze act that these guys have -- because mistakes and gaffes matter oh so much in these contests," said Tom Hollihan, a communications professor at USC's Annenberg School.

The two men will also have very different objectives in the three 90-minute presidential debates. McCain will try to convey his experience but must also show that "he has the energy and the forward-looking vision to lead" when matched with a much younger challenger, Hollihan said.