Other surveys show that a larger number 40 percent or more believe that Iraq had some role in the attacks, or that Iraq gave substantial support to al-Qaida.
The 9/11 Commission and intelligence experts never found such a connection, but even after six years of reports and news accounts, the myth of a strong Iraq-Sept. 11 link persists.
"It's astonishing those numbers are so high six years after 9/11, that so many people see a strong link," said John Mueller, a political scientist at Ohio State University who has studied attitudes on war and national security. "I would have expected
10 percent or so."
In part, the poll number is a reflection of what many people want to believe, several analysts said. Plus, there is an ongoing campaign by the Bush administration to promote a connection to sell the Iraq war.
"The Bush administration has been
extremely clever at suggesting connections without being explicit," Mueller said. "And if you support the war, you want to believe it. You hear soldiers in Iraq all the time say that they're there because of 9/11."
The most recent survey, a New York Times/CBS News Poll conducted last week, found that 33 percent of all Americans, including 40 percent of Republicans
27 percent of Democrats, believed Saddam was involved in the 9/11 attacks.
That number was much higher
53 percent in April 2003, the month after the U.S. invasion of Iraq, after weeks of warnings from the administration that Saddam was prepared to use weapons of mass destruction or give them to terrorists.
"That number has gone down, but it's still remarkably high," said Steve Kull, who has conducted polling on foreign policy attitudes as director of the Program on International Policy Attitudes at the University of Maryland.
Last year one of Kull's polls found that 14 percent believed Iraq was "directly involved" in the 9/11 attacks, and 35 percent said Iraq gave "substantial support" to al-Qaida. And 44 percent said they thought the Bush administration had said Iraq was directly involved in the Sept. 11 terrorist attacks.
A Zogby poll last year found that 46 percent believed "there was a connection between Saddam Hussein and the 9/11 attacks."
The Bush administration has tried several times to reframe the Iraq war as an integral part of the struggle against al-Qaida and Islamic extremism. Last month, President Bush said the insurgents killing U.S. in troops in Iraq "are the same ones who attacked us on Sept. 11."
He was trying to link the wider terrorist network of al-Qaida with al-Qaida in Iraq, which did not exist before the 2003 invasion.
"When you hear those phrases about Iraq and al-Qaida repeated over and over again," Kull said, "the association is hard to break."
In recent weeks a pro-Bush group, including former administration officials, launched a series of TV ads in support of the war that link Iraq to 9/11. In one ad, veteran John Kriesel, who lost both legs in Iraq, says, "They attacked us before and they will attack us again. They won't stop in Iraq."
When Kriesel says "they attacked us before," an image appears of the second airliner about to hit the World Trade Center on Sept. 11.
Sam Popkin, a professor at the University of California at San Diego, has studied attitudes on wars from Vietnam to the present.
"There's a desperate effort to keep saying about Iraq that these are the guys who hit the World Trade Center," he said.
Popkin, who has advised Democratic candidates, said the poll results show that six years after the attacks, many Americans may not grasp global realities.
"It probably reflects a belief that all Arabs who hate us must be friends," he said.
But the higher-than-expected poll numbers notwithstanding, recent polling also shows that a majority of Americans now differentiate the war on terrorism from the Iraq war. In the New York Times/CBS poll, 60 percent said that members of the Bush administration "intentionally misled the public" in making the case for the Iraq war.
Matthew Dowd, a chief strategist in the Bush re-election campaign in 2004, said he has seen a shift in attitudes about terrorism and Iraq.
"The war in Iraq is now seen exclusively as a foreign policy concern, and the American public no longer supports the initiative as part of national security," Dowd said in a blog posted on huffingtonpost.com.
"The dynamic has changed," Dowd concluded, "and most of the public sees no 'positive' relationship between the fight against terrorism and the war in Iraq."
Frank Davies can be reached at 202-662-8921.