WASHINGTON -- President Bush angrily rejected yesterday as "not credible" the findings of a new Johns Hopkins report putting the number of violent Iraqi civilian deaths at more than 600,000, twenty times what Bush has estimated.
Calling the study's methodology "pretty well discredited," Bush declined to reassert his 2005 claim that about 30,000 civilians had been killed in Iraq. But, he said, "I stand by the figure a lot of innocent people have lost their life."
White House officials forcefully disputed as pure guesswork the new report, published in the online edition of a leading British medical journal. They added that the timing - four weeks before the midterm elections - was politically motivated.
Democrats seized on the study by researchers at the Bloomberg School of Public Health, using data compiled from May to July, as a fresh opportunity to criticize Bush's handling of the war.
Sen. Edward M. Kennedy of Massachusetts called it "a chilling and somber reminder of the unacceptably high human cost of this war, standing in stark contrast to the administration's refusal to admit that Iraq is spiraling into a civil war."
The president, in a Rose Garden news conference, refused to name a figure for the total number of casualties in Iraq.
"I do know that a lot of innocent people have died, and that troubles me and it grieves me," he said.
Military officials brushed off the Hopkins figures, calling them strikingly higher than most other available estimates of civilian deaths.
Gen. George Casey, commander of U.S. ground forces in Iraq, said the report's estimate was "way, way beyond any number I have seen."
"I have not seen a number higher than 50,000," he said, "and so I don't give it that much credibility at all."
Iraq Body Count, a group that gathers fatality figures from news reports and posts them online, estimates civilian deaths since the 2003 invasion between 43,850 and 48,693.
The White House said it considered the Johns Hopkins group's figures "flawed" because its margin of error was huge - numbering in the hundreds of thousands. Aides noted that an earlier estimate the group released in October 2004, which used similar methods to conclude that 100,000 Iraqi civilians had died, drew criticism from a British official and research specialists who questioned its methodology. That report came out just days before the last presidential election.
U.S. officials "defer to the Iraqi Ministry on Health on the precise number of civilian casualties, but even the highest-end estimates put the figure at a very small fraction of what this study is reporting," said Frederick Jones, a White House spokesman.
The Lancet, which published the report, "seems to be a medical organization that has politicized itself," said Jones. "People need to give a lot deeper scrutiny to the origin of these figures and the motivation behind the organization that produces them."
In a conference call with reporters, Dr. Gilbert Burnham, lead author of the study, defended his findings and research practices, which he called standard among international health specialists.
"I feel very confident in the results," Burnham said.
The study was initially planned to be completed during the summer, he said, denying any political motive behind its release yesterday.
"Obviously, coming this close to the election, people are going to accuse us of having political motivations. But I'd like to assure people that our motivations here are entirely on the basis of science and our very great concern about citizens and civilians caught up in conflict situations," Burnham said.
To compile the study, researchers relied on door-to-door surveys in 47 randomly selected areas of Iraq - encompassing 1,849 households and 12,801 people - to calculate the change in death rates over the years since the U.S. invasion. Researchers extrapolated their results to the entire country, with a population of about 26 million.
Some scientists have cast doubt on the findings, pointing with skepticism to the wide statistical range - known as a "confidence interval" - used to report the results. The report set that range between 426,369 violent fatalities and 793,663, and researchers concluded that the highest probably was that the actual number of violent deaths was 601,027.
The researchers estimated that 654,000 more Iraqis died of all causes since the invasion than would have died in a comparable period before.
"The numbers are so radically different from anybody else's," said Michael O'Hanlon, a defense policy analyst at the Brookings Institution. "It doesn't square with logic."
Other analysts said the higher numbers ring true because U.S. and Iraqi officials and news media might have the capability to record only a tiny portion of the actual scope of fatalities in Iraq.
"The level of violence that we're seeing - and recognizing that what we're seeing is a small fraction of what's probably going on in the country - suggests that the numbers probably are higher than previously estimated," said James Dobbins of the RAND Institute.
"It's a reminder of the devastating impact that the conflict's having in Iraq." He noted that even Bush's earlier estimate of 30,000 deaths would be the Iraqi equivalent of "a 9/11 attack on Iraq every week since we arrived. These new figures are suggesting maybe it's not every week - maybe it's every day."
Democratic Sen. Jack Reed of Rhode Island, just back from a trip to Iraq, said the true estimate of Iraqi fatalities probably lies somewhere between what the Hopkins researchers found and what Bush has said.
"The level of violence is significant, and the civilian casualties are growing," he said. "The study today might be too high, but I think what I've seen publicly released - my inference is that they're probably too low."
Some analysts said the new numbers are unlikely to have much impact on the public's view of the war in the month leading up to the election, given historical evidence that Americans react much more strongly to U.S. death tolls than to foreign body counts.
"It's another piece of bad news" for Bush, said Richard J. Stoll, a Rice University political scientist. But, he said, "for most Americans, the thing that they use to judge 'How well are we doing? How well is the president doing? Do I approve of the president?' is the level of American casualties."
Still, Democrats said the report reflected Bush's mishandling of the war.
"Whether this estimate is high, low or the same as the actual numbers of deaths of Iraqi civilians, the fact is that chaos, misery and insecurity in Iraq are compounding the effects of the poor planning and bad decisions that the Bush administration continues to make in Iraq," Sen. Patrick J. Leahy, a Vermont Democrat, said in a statement.
Sun reporter David Wood contributed to this article.