Vice President Cheney yesterday accused critics of engaging in "revisionism of the most corrupt and shameless variety" in the Iraq debate, in a major speech that reflected the uncompromising style that has made him a touchstone for many of the controversies shadowing President Bush.

In remarks before the American Enterprise Institute, a conservative research organization where he once served as a research fellow and a trustee, Cheney said Democratic critics of the war are lying when they say Bush lied about prewar intelligence to justify the invasion of Iraq.

"Any suggestion that prewar information was distorted, hyped, fabricated by the leader of the nation is utterly false," Cheney said, decrying the "self-defeating pessimism" of many Democrats. He added that to begin withdrawing from Iraq now, as some lawmakers have suggested, "would be a victory for the terrorists."

The 19-minute speech cast the vice president in a familiar role: as the no-nonsense purveyor of a Bush administration policy that he was central in developing. Yet Cheney's defiant public image concerns even some White House aides.

The speech came amid a determined White House effort to answer critics of a war that polls show is growing increasingly unpopular, and that in recent weeks has helped erode Bush's standing with the public to the lowest of his presidency.

But the war has hurt Cheney's reputation even more. A recent Newsweek poll found that only 29 percent of Americans regard him as honest and ethical. The same poll found that more than one in four Republicans agreed with that dim assessment of Cheney's integrity -- a finding that surprised some top White House aides, who were already concerned about how the public views the vice president.

Beyond Iraq, Cheney's popularity is sagging under the weight of the indictment of his former chief of staff, I. Lewis "Scooter" Libby, in the CIA leak case and by his determined campaign to exempt the CIA from anti-torture standards, which has provoked opposition even from Republicans on Capitol Hill.

"I think he is the point man for the discontent with the Bush administration," said Andrew Kohut, director of the Pew Research Center for People and the Press. "There are many people who associate him with Bush policies, notably the war in Iraq, that they are very unhappy with."

Kohut said that Cheney's sinking popularity may be compounded by his political style. "He is not a jocular personality," Kohut said. "He's not out talking in the public a lot. Certainly, the Scooter Libby problem, which has really added to the White House woes, is associated with him. All of these things tend to make him a bigger heavy for Bush's critics."

One aide, who requested anonymity in order to discuss internal strategizing, said several White House officials were startled when Cheney replaced Libby -- who resigned after being indicted -- with two other close advisers who testified in the investigation and are known to share Cheney's views on detainee policy and the war. Many aides assumed that Cheney would at least nod to the controversy his office helped create by appointing people who are far removed from it.

One supporter said his strengths and liabilities are of a piece. "His numbers have dropped and he is probably not the best messenger to independents and swing voters, but in terms of managing an argument, he is good at it and his style lends itself to it," said William Kristol, editor of the conservative Weekly Standard.

Even those White House aides who expressed concern about Cheney in recent weeks said he remains the most forceful and articulate advocate of Bush's Iraq policy, especially with Defense Secretary Donald H. Rumsfeld often on the sidelines in the debate.

"Cheney's audience is people who care about substance, and when he talks they listen, and the base, who wants to see him fight, and when he fights they follow," said Mary Matalin, an informal Cheney adviser. She said what has hurt Bush is not Cheney but rather the president's failure to respond to allegations that he lied about Iraq's weapons.

In nearly five years as vice president, Cheney has offered one consistent piece of advice that has shaped -- and now threatens to stymie -- the Bush presidency: Don't get distracted by the hand-wringers and naysayers in Washington.

"You don't compromise on beliefs or decision-making when you are at a low ebb," a source close to Cheney said.

Cheney, for example, is scheduled to appear at a Dec. 5 fundraiser in Houston for Rep. Tom DeLay (R-Tex.), who was forced to step down as House majority leader after being indicted.

White House aides count Cheney as a major asset in the war debate because his speeches often command front-page attention and because he can explain the issue more forcefully and often more clearly than Bush. In addition, they said, his resolve energizes Bush's conservative base -- even if it antagonizes the president's political opponents.

"He certainly has been the point person on Iraq," said Karlyn H. Bowman, a resident fellow at the AEI who tracks public opinion. "When people think of Dick Cheney, they think about Iraq. He's the lightning rod."

During his speech, Cheney sought to draw a distinction between what he called legitimate debate over the war and "dishonest" and "reprehensible" charges that the administration twisted prewar intelligence to justify invading Iraq in March 2003. Cheney repeated assertions -- disputed by some senators -- that members of Congress had access to the same intelligence that was provided to Bush about the threat of Iraqi weapons of mass destruction under the rule of Saddam Hussein.

Shortly after Cheney's speech, Senate Democrats fired back, saying he missed an opportunity to set the record straight. "The vice president and this administration have a credibility problem," said Minority Leader Harry M. Reid (Nev.). "Rather than giving our troops a plan to move forward in Iraq and changing their failed course, they continue to ignore the facts and lash out at those who raise legitimate questions."

Although the United States has not found any stockpiles of banned weapons in Iraq, Cheney said, "I repeat that we never had the burden of proof; Saddam Hussein did."

Despite the mounting death toll and the problem in restoring basic infrastructure such as reliable electrical service in much of Iraq, Cheney said the United States is making progress as Iraqis move toward establishing a democracy and building a security force. He vowed that the United States will remain steadfast until the job is done.

"The terrorists . . . have contempt for our values, they doubt our strength, and they believe that America will lose its nerve and let down our guard," he said. "But this nation's made a decision: We will not retreat in the face of brutality, and we will never live at the mercy of tyrants or terrorists."

Some observers called into question Cheney's repeated description of the enemy in Iraq as "terrorists" who are seeking to control that country to establish a base from which they can "launch attacks and to wage war against governments that do not meet their demands."

U.S. intelligence agencies say foreign terrorists represent a minority of the insurgent forces; the vast majority are Iraqis. Classified findings by U.S. intelligence agencies are reflected in a study by Anthony H. Cordesman of the Center for Strategic and International Studies, released yesterday, which estimates that at least 90 percent of the fighters are Iraqi.