Former Vermont Gov. Howard Dean and Sens. John Kerry, Joe Lieberman and John Edwards took turns criticizing Clark, saying he was speaking warmly of Mr. Bush as recently as 2001, and more recently switched positions to oppose the war with Iraq.
"I did not vote for George Bush. I voted for Al Gore," Clark retorted Thursday in the most contentious of four debates to date in the battle for the Democratic nomination.
The field of Democratic contenders — shrunken by one with Florida Sen. Bob Graham's withdrawal from the race — met onstage at the Orpheum Theater in Phoenix, capital of a state that holds an early primary on Feb. 3.
When they weren't sparring with one another, Democrats took time to heap fresh criticism on Mr. Bush's postwar policy in Iraq, faulting him for failing to win significant help from other countries.
Clark — who voted for Republicans Richard Nixon, Ronald Reagan and George H. Bush for the White House — labored to fend off the criticism from the early moments of the debate. "I would never have voted for war. The war was an unnecessary war and it's been a huge strategic mistake for the country," he said.
But Dean said that exactly one year ago, Clark had advised a Democratic congressional candidate in New Hampshire to vote for legislation authorizing the war in Iraq — a war the former general now criticizes sharply.
Lieberman, a supporter of the conflict, jabbed at Dean and Clark simultaneously. In a backhanded compliment, he said Dean had been steadfast in his opposition to the war.
By contrast, he criticized Clark for what he called a history of inconsistency on Iraq. He said Democrats need a candidate who can "reach a conclusion and stick to it."
Dean accused several rivals of giving Mr. Bush "a blank check to go to war in Iraq" by voting for or voicing support for a congressional resolution last year.
But Dean also said he would support Bush's request for $87 billion to maintain the troops stationed in Iraq and help rebuild the country.
That, in turn, drew a challenge from Ohio Rep. Dennis Kucinich, who said the troops should be brought home because "they are targets" for terrorists in the land that Saddam Hussein once ruled.
Former Illinois Sen. Carol Moseley Braun agreed that help from the United Nations as well as NATO is necessary, but said the United States has an obligation to rebuild Iraq since it waged war there.
Rev. Al Sharpton said Mr. Bush needs to admit he made a mistake in attacking Iraq without U.N. support. "The president went to the U.N. and said, "Help us on my terms.' If I were president, I would go in and say, 'We were wrong,'" he said.
With the pace of the campaign quickening, the Democrats traded jabs over economic policy as well.
Gephardt told a questioner in the debate audience he favors repealing Mr. Bush's tax cuts in their entirety, and insisted that would not result in an increase in her taxes. He said the proceeds would fund his health care plan.
But Kerry, who favors retaining Mr. Bush's tax cuts only for middle-income individuals, said, "You're going to pay more tax" if all cuts are repealed.
The attacks on Clark represent a shift in the tone of the race. At his first debate after joining the field, Clark was largely ignored, and most of his rivals spent time attacking the one-time front-runner Dean.
But the retired Army general has jumped to a lead in some national polls within days of his entry into the race in September.
At the same time, developments since then have made Clark look more vulnerable. His campaign manager quit — apparently over divisions between Clark's grassroots supporters and political pros — and the retired general said he would return speaking fees that some claimed violated federal campaign finance laws.
Edwards spoke dismissively of Clark, saying that he opposed Mr. Bush from the start — even "when some on this stage had hope for" him.
Kerry said Clark had praised Mr. Bush at a Republican fund-raiser last year — at a time he said the Bush administration had already won tax cuts for the rich from Congress and was trying to tap into the Arctic National Wildlife Refuge in Alaska for oil.
"I could still have hope in early 2001 that this administration would learn its lessons, as most administrations do," said the retired general.
"Americans believed they had selected a compassionate conservative," he added. "Instead we had a guy who has deepened the deficits. He's taken us recklessly into war. And he's been a radical, not a compassionate conservative."
Clark also criticized his Washington-based rivals for failing to take action against Mr. Bush's foreign policy. He said North Korea and Iran are accelerating their nuclear weapons development in reaction to the administration's "pre-emptive doctrine" and the Democrats in Congress are doing nothing to stop it.