"The president may not want to admit mistakes, but his choices in Iraq have so far produced a tragedy of errors," he said. "Staying the course does not mean stubbornly holding to the wrong course."
In the address, taped in Pittsburgh, on Friday, he added: "The failure of the administration to internationalise the conflict has lost us time, momentum and credibility and made America less safe."
The BBC's Jannat Jalil, in Washington, says this is an attempt by John Kerry, just seven months before an election that's too close to call, to show how he would deal with the mounting violence and casualties in Iraq.
The volatile situation there could become one of the most important issues in this year's presidential election campaign. This is a big concern for the current administration as the latest opinion polls indicate public support for Mr Bush's handling of Iraq is slipping.
But it is hard to see countries sending troops to Iraq given the current violence there and President Bush has already embraced the plan that would give the United Nations a more prominent role in Iraq, our correspondent adds.