The June 20-21 poll comes amid turmoil in Iraq, as fighters from the Sunni group Islamic State of Iraq and Greater Syria (ISIS) are reportedly beheading victims in a bid to instill fear as they sweep through the country's north.
Republicans, independents and Democrats back the withdrawal decision less now
In response to the threat of attacking the capital city of Baghdad, Obama has sent 275 military troops to help secure the US embassy and 300 military advisors to assist the Iraqi government.
While the majority of Americans still back the 2011 decision to leave Iraq and 57 percent believe the war was a mistake, the move is being heavily scrutinised in light of the current situation, Xinhua reported citing the poll.
All three partisan groups - Republicans, independents and Democrats - are less likely to back the withdrawal now as they were in 2011.
Independents showed the greatest change in opinion, with approval falling by 18 points. Approval among Democrats and Republicans also decreased significantly, by nine and 10 points respectively, Gallup found.