Obama said he would limit abortions in the late stages of pregnancy if there are exceptions for the mother's health. He said he knew that people who consider themselves pro-life will find his stance "inadequate."
He said the government should do more to prevent unwanted pregnancies and to help struggling new mothers, such as providing needed resources to the poor, and better adoption services.
McCain expressed his anti-abortion stand simply and quickly, saying human rights begin the instant a human egg is fertilized.
McCain, who adopted a daughter from Bangladesh, also called for making adoption easier.
The men's comments came at a two-hour forum on faith hosted by the minister Rick Warren at his megachurch in Orange County, Calif.
Obama joined Warren for the first hour, and McCain for the second.
The two candidates briefly shook hands and hugged each other during the switch. McCain said he did not see or hear Obama's session, which would have given him an advantage.
Obama said America's greatest moral failure is its insufficient help to the disadvantaged. He noted that the Bible quotes Jesus as saying "whatever you do for the least of my brothers, you do for me." He said the maxim should apply to victims of poverty, sexism and racism.
McCain said the nation's greatest moral shortcoming is its failure to "devote ourselves to causes greater than our self-interests."
After the September 2001 terrorist attacks, McCain said, there should have been a national push for joining the Peace Corps and other volunteer organizations. His comment seemed an indirect criticism of President Bush, who had urged tax cuts and more shopping at the time to stimulate the economy.
McCain also said he would pursue Osama bin Laden "to the gates of Hell," another goal that might be seen as a swipe at the Bush administration.
Both men said marriage is a union between a man and a woman. Obama added that he supports civil unions for gay partners, which would give them rights such as hospital visits with one another. He said he opposed a constitutional ban on gay marriage, calling the matter a state issue.
McCain's answer was less clear. If a federal court ordered his state, Arizona, to honor gay marriages allowed in Massachusetts, he said, "then I would favor a constitutional amendment. Until then, I believe the states should make the decisions within their own states."
In several cases, Obama gave a Christian interpretation to his generally liberal political views. He said he is redeemed by Jesus, who died for his sins.
McCain tended to give shorter, less complex answers, winning somewhat more applause than Obama from the large, evangelical church's audience. On domestic matters, he restated his call to "drill now" in U.S. lands and waters for oil and natural gas.
McCain, asked the toughest decision in his life, cited his refusal to be released ahead of fellow U.S. prisoners of war in North Vietnam. "It took a lot of prayer," he said.
He retold his story of a Christmas Day celebration outside his cell, when a prison guard etched a cross into the dirt. "For a moment, we were just two Christians worshipping there," McCain said.
Warren asked each man to name a Supreme Court justice he would not have appointed. Obama cited Clarence Thomas. "I don't think that he was a strong enough jurist or legal thinker at the time for that elevation, setting aside the fact that I profoundly disagree with his interpretations of a lot of the Constitution," Obama said.
He also named Justices Antonin Scalia and John Roberts, although he praised their intellect.
McCain named the court's four most liberal members: Ruth Bader Ginsburg, Stephen Breyer, David Souter and John Paul Stevens.
When Warren asked Obama to define the word "rich," the Illinois senator teased the pastor about the mammoth sales of his book, "The Purpose Driven Life." Obama noted his plan to add a new Social Security payroll tax to incomes above $250,000 a year.
McCain said, "some of the richest people I've ever known in my life are the most unhappy."
He said being rich should be defined by having a home and a prosperous and safe world. Without mentioning Obama, he said some want to increase taxes.
"I don't want to take any money from the rich. I want everybody to get rich," McCain said.
When pushed on an exact number, he joked: "If you're just talking about income, how about five million?" He added, "I'm sure that comment will be distorted."
Asked to name three wise people they would listen to, Obama named his wife, Michelle; his maternal grandmother, who lives in Hawaii; and, not limiting himself to only a third, named several Democratic and Republican lawmakers.
McCain named Gen. David Petreaus, head of U.S. troops in Iraq; U.S. Rep. and veteran civil rights leader John Lewis, D-Ga.; and former eBay CEO Meg Whitman, a top adviser to his campaign.
He lauded Whitman for turning a five-person business into a billion-dollar piece of the economy. "It's one of these great economic success stories," McCain said.
Obama, asked his most significant policy shift in the last 10 years, cited welfare reform. As an Illinois state senator, he worked to mitigate what he thought could be "disastrous" effects of President Clinton's welfare reform effort. But over time he said he came to embrace Clinton's approach.
"We have to have work as a centerpiece of any social policy," Obama said.
Asked why they want to be president, Obama said the United States should be an empathetic power for good in the world, a mission he fears is slipping away.
McCain said, "I want to inspire a generation of Americans to serve a cause greater than its self interest...But I also believe we face enormous challenges, both of national security and domestic."
