McCain said the Obama administration never had serious negotiations with Iraqis to keep residual forces in the country and cited Shiite cleric Moqtada Sadr's threat to target embassy personnel once troops leave.
"We should have backed (former Prime Minister Iyad) Allawi a lot more. We should have become engaged back then after their elections, which, frankly, due to our ambassador at the time and the State Department, we didn't engage," the 2008 GOP presidential nominee said in an interview on ABC's "This Week."
McCain said a full withdrawal of U.S. troops from Iraq could leave the door open for Iranian influence in the unstable region.
"In my view, this can lead to Iranian influence in Iraq through Sadr, and (Iraqi Prime Minister Nouri al-) Maliki, I think, is leaning more and more towards alliances with Iran," he said.