The poll was taken over last week, during which the mounting US casualties were denting the public's confidence in the Bush administration's grasp on events. The increasing cost of the occupation has further added to disillusionment.
The Washington Post-ABC poll released yesterday shows the number who approve of Mr Bush's handling at 47%, a fall of 28% percentage points on the end of April.
Those who disapproved had climbed to 51%, the first time the figure has broken 50 since the war began.
The poll showed the unity displayed after September 11 and again at the beginning of the war almost completely dissipated, giving way to a public increasingly split on partisan lines.
Some 54% said the war was worth fighting and 44% said it was not. In March the figures were 70% and 27% respectively.
One year away from the presidential election, the poll showed Mr Bush only marginally ahead of any Democratic candidate and with his approval ratings on the economy and healthcare also falling.
The president, at his Texas ranch, was informed of the attack on the Chinook helicopter by staff, "and will get updates as required", said a White House aide, Trent Duffy.
The administration continued to sound upbeat about the occupation: it offered its condolences to the families of the bereaved, but insisted there would be no change in course.
The defence secretary, Donald Rumsfeld, said the loss was "a tragic day for America and for these young men and women. In a long, hard war, we're going to have tragic days, as this is. But they're necessary. They're part of a war that's difficult and complicated. And in the last analysis, the people who are firing off these surface-to-air missiles are the same people who are killing Iraqis. We can win this war. We will win this war. It is tough. It is going to take time."
With Mr Bush's declaration of victory on May 1 already looking like a political liability, both the White House and the Pentagon are keen to play down expectations that the situation will be brought under control quickly.
Lieutenant-General Ricardo Sanchez, the US commander in Iraq, was quoted in the Washington Post yesterday describing the Iraqi resistance as a "determined enemy".
"Are we going to be able to eliminate it all?" he asked. "Probably not."
But the most senior Democrat on the Senate foreign relations committee, Joseph Biden, said yesterday that the US might need to send more troops and go back to the international community for more help.
"In the short term, we may need more American forces in there while we're training people up," he said. "And we have to be prepared to go back to our European friends and say: 'We need more help, we are willing to give you more say.'"
Iraq's six neighbours yesterday condemned terrorist attacks against civilians and said they would secure their borders, amid US charges that foreign militants were behind the wave of violence.
But the foreign ministers of Syria, Iran, Turkey, Saudi Arabia, Kuwait, and Jordan, as well as Egypt which is not a neighbour, made no mention of the increasing attacks on military targets.
"[The ministers] condemn the terrorist bombings that target civilians, humanitarian and religious institutions, embassies and international organisations working in Iraq," their closing statement said after a two-day security meeting in Damascus.
