"Yet today, President Bush tells us that he would do everything all over again, the same way. How can he possibly be serious?" Kerry said at New York University.
Mr. Bush retaliated quickly, accusing his Democratic rival of leaving behind a thicket of contradictory positions on the war.
"Today my opponent continued his pattern of twisting in the wind," Mr. Bush said at a rally in New Hampshire. "He apparently woke up this morning and has now decided, no, we should not have invaded Iraq, after just last month saying he would have voted for force even knowing everything we know today."
Kerry, a fourth-term Massachusetts senator, voted to give the president authority to wage the war and he said in August he still would have voted that way had he known there were no weapons of mass destruction in Iraq.
The Democratic presidential candidate makes a distinction between granting a president war-making authority as a member of the Senate and, as commander in chief, actually taking that fateful step. Republicans have accused Kerry of waffling on the war.
Kerry said Monday, "Is he really saying that if we knew there were no imminent threat, no weapons of mass destruction, no ties to al Qaeda, the United States should have invaded Iraq? My answer is no because a commander in chief's first responsibility is to make a wise and responsible decision to keep America safe."
"Saddam Hussein was a brutal dictator who deserves his own special place in hell," Kerry said. "But that was not, in itself, a reason to go to war. The satisfaction we take in his downfall does not hide this fact: We have traded a dictator for a chaos that has left America less secure."
Kerry's speech was timed one day ahead of Mr. Bush's scheduled address to the U.N. General Assembly in New York. The president planned to strike back at Kerry's increasingly aggressive criticism on Iraq, aides said.
Kerry said Monday, "The president's insistence that he would do the same thing all over again in Iraq is a clear warning for the future. And it makes the choice in this election clear: more of the same with President Bush or a new direction that makes our troops and America safer."
He said the president's invasion of Iraq has created a crisis that could lead to unending war and raises questions about whether Mr. Bush's judgment is up to presidential standards. He offered his own four-point plan starting with pressing other nations for help.
The president's mistakes, Kerry said, "were not the equivalent of accounting errors. They were colossal failures of judgment — and judgment is what we look for in a president."
Kerry contended that Mr. Bush has not been honest about the war's rationale or costs. He said the president's decision to go to war against Iraq has distracted from a greater threat to the United States — more terrorist attacks.
"In Iraq, this administration has consistently over-promised and underperformed. This policy has been plagued by a lack of planning, an absence of candor, arrogance and outright incompetence. And the president has held no one accountable, including himself," Kerry said.
With six weeks remaining until Election Day, the Massachusetts senator was pressing the debate on an issue that has given him trouble in his bid for the White House.
The Republicans have accused him of staking out unclear, even contradictory, positions on Iraq. His speech was aimed at explaining his stance and drawing clear differences with Mr. Bush's leadership at a time when troubles in Iraq are mounting.
Kerry tried to turn the criticism back against the president by pointing to varying administration arguments for going to war.
"By one count, the president offered 23 different rationales for this war," Kerry said. "If his purpose was to confuse and mislead the American people, he succeeded."
Kerry said Mr. Bush's two main rationales — weapons of mass destruction and a connection between al Qaeda and the Sept. 11 attacks — have been proven false by weapons inspectors and the bipartisan commission investigating the attacks.
"Only Vice President Cheney still insists that the earth is flat," said Kerry.
"This president was in denial," Kerry said. "He hitched his wagon to the ideologues who surround him, filtering out those who disagreed, including leaders of his own party and the uniformed military. The result is a long litany of misjudgments with terrible consequences."
Kerry's new Iraq strategy got a boost of sorts over the weekend, with several influential Republican senators raising questions about the violence and turmoil in Iraq.
"The fact is, a crisp, sharp analysis of our policies is required. We didn't do that in Vietnam, and we saw 11 years of casualties mount to the point where we finally lost," said Sen. Chuck Hagel, R-Neb., a Vietnam War veteran who is co-chairman of Mr. Bush's re-election committee in Nebraska. He spoke on CBS News' "Face the Nation."
Sen. John McCain, R-Ariz., another Vietnam War veteran, was asked on "Fox News Sunday" about Bush's often-rosy pronouncements about progress in Iraq. McCain said Mr. Bush was not being "as straight as we would want him to be" about the situation.