With the first primary only three days away, the former first lady had little time to made the case she hoped would knock Obama off stride.

Challenged on health care, Obama acknowledged that he has said if he were designing a system from scratch, he would set up a single-payer system that would give coverage to all. He said that is impractical, given the current system in which so many people receive their insurance from employers.

Obama's health-care plan relies on government financial incentives and cost-cutting to help the uninsured afford coverage. But unlike Clinton and Edwards, he does not require adults to buy coverage or pay a penalty if they fail.

"I disagree with that because as I go around, I don't meet people who avoid getting health care. The problem is they can't afford it," he said.

The opening moments of the debate produced agreement on Obama's summertime statement, controversial at the time, that he would take action against terrorist leader Osama bin Laden if he had actionable intelligence that he was hiding in Pakistan and the government there did not act.

"As much as possible," the United States should seek agreement from Pakistanis, Obama added.

Edwards agreed. "If I as president of the United States know where Osama bin Laden is I would go get him," the 2004 Democratic vice presidential nominee said.

Likewise, New Mexico Gov. Bill Richardson said that if diplomacy failed and the Pakistani government was incapable of moving against the terrorist leader, "then you do take that action."

Clinton agreed, saying, "At some point, probably when the missiles have been launched, the Pakistani government has to know they are on the way." She said that was important to make sure Pakistan didn't jump to the conclusion that it was under attack from India, its longtime rival in southwest Asia.