Social Security was one of the issues hotly debated in the 90-minute encounter in Tempe, Arizona, which is the only debate devoted to domestic issues.

Bush has proposed to fix the system by letting young people put some of the money collected to pay benefits into private savings accounts, but critics say the plan would need an extra trillion dollars over the next 10 years.

In defending his proposal, Bush said he believes that younger workers ought to be allowed to take some of their own money and put it into a personal savings account "because I understand that they need to get better rates of return than the rates of return being given in the current Social Security Trust."

Bush said the reform of Social Security would be "a vital issue" in his second term, if he got re-elected. "It is an issue that I'm willing to take on. And so I'll bring Republicans and Democrats together," he said.

Speaking of controlling the cost, Bush said: "I want to warn my fellow citizens, the cost of doing nothing, the cost of saying the current system is okay far exceeds the cost of trying to make sure we save the system for our children."

However, Kerry said Bush's plan to allow young people to take money out of Social Security and put it in their own accounts was "an invitation to disaster."

Kerry cited a report by the bipartisan Congressional Budget Office that Bush's plan would create a 2-trillion-dollar hole in Social Security. "They said that there would have to be a cut in benefits of 25 to 40 percent," he said.

"Now, the president has never explained to America, ever, hasn't done it tonight, where does the transitional money, that 2 trillion dollars, come from?" he said.

Kerry said he would protect Social Security by restoring fiscal responsibility. "I will not privatize it. I will not cut the benefits. And we're going to be fiscally responsible, and we will take care of Social Security," he said.

As for the remarks made by Alan Greenspan, the chairman of the Federal Reserve, that the reform of Social Security calls for either a cut in benefits or a raising of retirement age, Kerry said he would choose to fix the Social Security.

Kerry said the cancellation of Bush's tax-cut that went to the top 1 percent of America would save Social Security until the year 2075. If there were troubles later on, "we'll do exactly what we did in the 1990s, and we'll make whatever adjustment is necessary," he said.

"But the first and most important thing is to start creating jobs in America," he added.