They eased off the attacks after Obama's South Carolina win and clearly decided it was better to stay positive in Thursday's debate.
The pair burst into smiles at the suggestion they run for president and vice president on an "Obama-Clinton or a Clinton-Obama ticket."
"Obviously there's a big difference between those two," Obama said, adding how much he respected the former first lady. "I'm sure Hillary would be on anybody's short list."
"DAY ONE"
Obama, an early opponent of the Iraq war, questioned her 2002 Senate vote to authorize the war. He assailed Clinton's frequent comment she has the experience to lead from "day one" in the White House.
"Part of the argument that I'm making in this campaign is that it is important to be right on day one," he said.
Both candidates pointed to health care as one of their biggest policy differences. Clinton's plan requires all Americans to have coverage, and she criticized Obama's plan because it could leave up to 15 million people uninsured.
"You have to bite this bullet, you have to say 'Yes, we will try to get to universal health care'," she said.
Obama said anyone who wanted health care could get it under his plan, which would focus on bringing down costs.
The debate was their first since Obama crushed Clinton and John Edwards in a South Carolina landslide on Saturday, driving Edwards from the race. Both Clinton and Obama praised Edwards in their opening statements.
Earlier on Thursday, popular California Gov. Arnold Schwarzenegger endorsed McCain, boosting the Arizona senator's drive to gain his party's nomination for the White House.
"He is a great American hero and an extraordinary leader. This is why I am endorsing him to be our next president of the United States," the actor-turned-politician said.
California is the largest prize among the states that hold nominating contests on "Super Tuesday." McCain, the Republican front-runner, already leads state polls over former Massachusetts Gov. Mitt Romney.
Obama reported he raised $32 million (16 million pounds) in the month of January alone, matching his biggest three-month fundraising haul of the year and helping him pay for new ads in a half-dozen states that hold contests after "Super Tuesday."
