US Democrat presidential candidate John Kerry has made his most outspoken attack on President George W Bush over the conflict in Iraq.
Kerry is now focusing on the Iraq conflict and its impact on the US
In a speech setting out his plans for Iraq's future, he accused Mr Bush of "colossal failures of judgement".
President Bush immediately hit back, accusing Mr Kerry of again shifting his positions for political reasons.
The new focus on Iraq just six weeks before the 2 November poll holds risks for both men, correspondents say.
By tapping into public concern over violence in Iraq as an anti-war candidate, Mr Kerry could be accused of political opportunism, the BBC's Rob Watson in Washington says.
For President Bush - who insists things are getting better in Iraq - the risk is that the voters may not share his assessment, especially if the violence gets any worse, our correspondent adds.
Analysts describe Mr Kerry's apparent shift away from the economy as a bold but possibly reckless move.
Previously, the senator had been vague about whether or not he thought overthrowing Saddam Hussein was the right thing to do, the BBC's Justin Webb in Washington says.
Bush accused Kerry of flip-flopping on the Iraq issue
But on Monday, Mr Kerry - who voted to give President Bush the authority to go to war - said he would not have invaded.
He criticised the president for saying he would still have gone to war knowing that Saddam Hussein did not possess weapons of mass destruction.
"President Bush tells us he would do it everything all over again, the same way. How can he possibly be serious?" he said.
Mr Kerry said the president's decision to go to war had distracted from a greater threat to the US - more terrorist attacks - and created a crisis which could lead to an unending war.
"Iraq was a profound diversion from that war and the battle against our greatest enemy, Osama Bin Laden and the terrorists," he said in his speech delivered at New York University.
"Invading Iraq has created a crisis of historic proportions and, if we do not change course, there is the prospect of a war with no end in sight.
"Saddam Hussein was a brutal dictator who deserves his own special place in hell. That was not a reason to go to war. We have traded a dictator for a chaos that has left America less secure."
He also reminded his audience that more than 1,000 Americans had now died since the start of the war.
He offered his own four-point plan for handling the conflict:
- Get more help from other nations
- Provide better training for Iraqi security forces
- Provide benefits to the Iraqi people
- Ensure democratic elections can be held next year as promised.
These measures could mean US troops coming home over the next four years, beginning next summer, Mr Kerry said.
President Bush hit back immediately at Mr Kerry's strong criticism.
Mr Bush told a campaign rally in Derry, New Hampshire, that it was just another case of his challenger reversing policy for political ends.
"Last December he [Mr Kerry] said this, and I quote, 'those who doubted whether Iraq or the world would be better off without Saddam Hussein and those who believe we are not safer with his capture don't have the judgement to be president or the credibility to be elected president'".
"I could not have said it better," Mr Bush added.
The Bush campaign has been accusing Mr Kerry of inconsistency on Iraq, saying a change in the middle of the war was not what the nation needed.
Bush campaign spokesman, Steve Schmidt, said Mr Kerry's goal of pulling US troops out of Iraq in his first term sent "a clear signal of defeat and retreat to America's enemies that will make the world a far more dangerous place".