The candidates met briefly between interviews. Obama greeted McCain with a handshake and hug.
They did strike some common themes, such as the importance of rising above self-interest to serve one's country. But they also offered starkly different answers to Warren's question: "At what point does a baby get human rights?"
Obama said: "I think that whether you are looking at it from a theological perspective or a scientific perspective, answering that question with specificity, you know, is above my pay grade." He added that he supports the landmark decision Roe vs. Wade but said the issue has "moral and ethical content" and stressed his commitment to reducing the number of abortions.
McCain, however, immediately responded that a baby's rights begin at conception. Perhaps seeking to tamp down alarm among conservatives over his recent comment that he's open to a running mate who favors abortion rights, he continued: "I will be a pro-life president, and this presidency will have pro-life policies."
After sustained applause, Warren quipped: "OK, we don't have to go longer on that one."
Though the candidates came down on opposite sides of the California initiative that would ban gay marriage, both stressed that they opposed same-sex marriage. Obama called marriage "a sacred union," drawing applause when he added, "God is in the mix."
Obama and McCain gave sharply divergent answers on which justices they would not have nominated to the Supreme Court.
Obama named Clarence Thomas, who he said was not a "strong enough jurist or legal thinker," and Antonin Scalia, though he said he didn't doubt "his intellectual brilliance."
McCain ticked off the four liberal members: Ruth Bader Ginsberg, John Paul Stevens, David H. Souter and Stephen G. Breyer.
Aides to both candidates said the forum was a rare chance for them to talk about their faith and to use Warren's network to reach out to evangelicals. The pastor's weekly services draw about 22,000 people. The forum was broadcast live on three cable networks.
This year, both McCain and Obama have faced challenges with religious communities.
Conservative Christian leaders have urged McCain to spend more time talking about his opposition to abortion and his promise to appoint conservative judges. Obama has continued to battle unfounded rumors that he is a Muslim.
