Democratic Contenders Step Up Attacks in Debate

"His plan would leave 15 million Americans out," she said. "That's about the population of Nevada, Iowa, South Carolina and New Hampshire." Those are the first four states that will vote in the Democratic presidential race next year.

Obama countered that the principal difference in their plans is that Clinton's mandates that all Americans buy health insurance while his does not. "I don't think that the problem with the American people is that they are not being forced to get health care," he said. "The problem is, they can't afford it. And that is why my plan provides the mechanism to make sure that they can."

The difference between the plans is significant. Obama decided not to include a mandate and instead has said he would seek to make insurance more affordable. Clinton, too, offers incentives to individuals and to small businesses to make insurance affordable.

Many health-care policy experts say that, without some kind of mandate, it will be difficult to achieve universal coverage. Obama's policy advisers have said that they believe his plan ultimately will provide insurance for everyone, but that if it falls short, he would be prepared to include a mandate at a later point.

On foreign policy, the candidates dealt with questions on Pakistan, Iraq and Iran. The lengthiest discussion was about how the United States should deal with Pakistani President Pervez Musharraf, who has declared a state of emergency, rounded up dissenters and put opposition leader Benazir Bhutto under house arrest. All said the United States should increase pressure on him, but they differed on how far to push him.

This led to a debate about whether there are times when human rights should take precedence over national security. New Mexico Gov. Bill Richardson said he would condition aid to Pakistan as a way of asserting the preeminence of human rights. Obama said the two are complementary, but Clinton and Sen. Christopher J. Dodd (Conn.) said national security should always be the first priority of any president.

The candidates took up the issue of Iran during a session that included questions from the audience but restated their long-standing positions, which find Clinton at odds with her rivals over a Senate resolution labeling the Iranian Revolutionary Guard Corps a terrorist organization.

Toward the end of the debate, Obama and Clinton got into another argument over Social Security. Obama, in response to a voter's question about looming retirements in the baby-boom generation, said he would consider raising the amount of wages that are subject to payroll taxes, currently $97,500. Clinton has said she would not propose any specific ideas for Social Security other than restoring fiscal discipline to the federal budget.

But after Obama's statement, she criticized a simple raising of the cap, saying she did not want to save Social Security on the backs of the middle class. "If you lift the cap completely, that is a $1 trillion tax increase," she said. "I don't think we need to do that."

Obama delivered a stern response, saying that only 6 percent of taxpayers earn more than $97,000.

"It is the upper class," he said. "You know, this is the kind of thing that I would expect from Mitt Romney or Rudy Giuliani, where we start playing with numbers. We start playing with numbers in order to try to make a point. And we can't do that."

Clinton took issue with his interpretation, saying she represents firefighters in New York who would find raising the cap burdensome.

Clinton talked at length about her role as a female candidate, saying she had not played the "gender card." Pressed to explain why, in recent weeks, she had described the campaign as a "boys' club," Clinton paused and looked at the questioner. "Well, it is clear, I think, from women's experiences that from time to time, there may be some impediments," she said.

Asked whether she is exploiting her status as a woman, Clinton said: "I'm not exploiting anything at all. I'm not playing, as some people say, the gender card here in Las Vegas. I'm just trying to play the winning card." She added: "People are not attacking me because I'm a woman. They're attacking me because I'm ahead."