NEW YORK -- Massachusetts Sen. John F. Kerry yesterday said President Bush's stubbornness in refusing to share authority and decision-making with the United Nations and other countries has put U.S. forces at greater risk, unduly burdened American taxpayers and made success in Iraq far more difficult.

"I think the approach of this administration has been consistent and stubborn in the way that it persists in this American occupation and in proceeding down its own road," the presumptive Democratic presidential nominee said. "It has made that mistake from Day One, and it is costing us money, and I think it is costing us lives."

In his most extended comments about Iraq since the eruption of new violence there that has left more than 80 Americans soldiers dead this month, Kerry charged that Bush failed in his news conference Tuesday night to offer a clear exit strategy for U.S. forces or to show any willingness to cede authority to gain greater international cooperation.

Kerry said withdrawal of U.S. forces should be determined by whether Iraq has been stabilized, not by whether it has achieved democracy. Democracy "shouldn't be the measurement of when you leave," he told reporters at an afternoon news conference. "You leave with stability. You hope you can continue the process of democratization -- obviously, that's our goal. But with respect to getting our troops out, the measurement is the stability of Iraq."

Kerry said more international support would help take the focus off the U.S. occupation. "The minute you have that international acceptance, you begin to reduce some of the capacity of people to focus on the 'infidel' United States and to focus their energies on our occupation alone."

As Kerry stepped up his public criticism of Bush on Iraq, the president's reelection campaign struck back hard. Former Montana Gov. Marc Racicot, the Bush campaign chairman, accused Kerry of a "cynical and defeatist" approach and of "very, very seriously undermining" the U.S. effort in Iraq and U.S. forces fighting there.

Racicot, who was joined in his conference call by former Defense Secretary Caspar Weinberger, said Kerry was attempting "to cause division and erode confidence" and said the senator's comments were evidence of "why he shouldn't be president."

Kerry denied he was making any effort to politicize the war but stood firm in his critique of the administration. "American soldiers are bearing the huge majority, the lion's share, of the risk in Iraq," he said. "It doesn't have to be that way; it never had to be that way."

The sharp exchanges underscored the growing debate over Bush's policies in Iraq and signaled the Democrats' belief that the president may be vulnerable on national security issues long presumed to be his greatest asset politically.

During a town hall meeting at City College in New York, Kerry was confronted by Walter Daum, a retired mathematics teacher at the college, who said the United States should withdraw immediately, angrily accusing the senator of supporting an imperialist war and of having the same policy as the president.

Kerry took issue with that characterization but said the United States could not cut and run. "I have consistently been critical of how we got where we are, but we are where we are, sir," he said. "And it would be unwise beyond belief for the United States of America to leave a failed Iraq in its wake."

Kerry said "it may take a new president" to bring about the policy changes that he sees as necessary to assure success and reduce the risk to U.S. forces in Iraq and that, if he were president now, he would be deeply engaged with other foreign leaders to spread the burden and the risk. He noted that the president had said he was counting on the help of U.N. special representative Lakhdar Brahimi to help arrange the terms of the transfer of power back to the Iraqi people June 30, but that there is still a fundamental difference between Bush's approach and his own.

"Why doesn't the president just come out and say, 'I want the U.N. to be a full partner, and the resolution that we pass will turn the authority over to them?' " he asked.

The exchange over Iraq overshadowed an announcement by Kerry's campaign of an expanded plan to put 500,000 young Americans in service to the country.

For those who give two full years of service, Kerry would have the federal government pay the cost of four years of in-state college tuition. A campaign fact sheet said Kerry could pay for the plan, whose cost the campaign estimated at $13 billion over 10 years, by ending the guaranteed profit for banks on student loans.

Kerry also continued to add millions to his campaign treasury, raising $6.5 million at two fund-raisers last night, after a $4.1 million event in Boston on Tuesday night.