A Washington Post-ABC News poll showed that out of 1002 adults who took part in the telephone survey, 52% said the US-led war did not contribute to the long-term security of their country, which was a chief premise on which US President George Bush justified the invasion of Iraq.
The poll, published on Wednesday, showed only 47% of Americans saying the war was keeping them safe from terrorism at home.
It was the first time a majority rejected Bush's contention. In late 2003, 62% supported his premise and 52% said the same three months ago.
By a very slim majority of 50%-49%, Americans also disapproved of how Bush was handling the campaign against terrorism, down from 56% in April.
Bush's overall disapproval rating was 52%, the highest of his presidency and two points higher than in April, the survey also said.
The Republican-led Congress also got bad marks, with 56% of Americans disapproving of the way Republican lawmakers were handling their job. But Democrats fared no better, also getting a 56% disapproval rating.