Sen. John Kerry accused President Bush Thursday night of a “colossal error in judgment” by ordering the invasion of Iraq. “The world is better off without Saddam Hussein,” the president shot back in campaign debate, adding his rival once said so himself.

“I agree with him,” the president added sarcastically, emphasizing his campaign’s contention that Kerry is prone to flip-flops.

In a 90-minute debate dominated by a war that has claimed more than 1,000 American lives, Kerry called the conflict a diversion in the broader struggle against terror and the hunt for Osama bin Laden.

Bush swiftly returned to his theme of Kerry as a man who changes his mind too often to be president.

“He voted to authorize the use of force and now says it’s the wrong war at the wrong time. …. I don’t think you can lead if you say wrong war, wrong time, wrong place. What message does that send to our troops?” said the Republican incumbent.

More than 1,000 Americans have been killed in Iraq since the U.S.-led invasion in March 2003, many of them by insurgents battling American forces. Not long before Bush and Kerry strode on stage, U.S. and Iraqi forces launched a major attack against the insurgents in Samarra. The U.S. command said government and police buildings had been secured in the city.

Both men used well-rehearsed lines during their face-to-face encounter, but this was the first time each had to listen to the criticism at close quarters.

Bush appeared perturbed when Kerry leveled some of his charges, scowling at times and looking away in apparent disgust at others. Kerry often took notes when the president spoke.

There were few light moments, the most prominent when they exchanged courtesies about their 20-something daughters. “Trying to put a leash on them,” Bush quipped of twins Jenna and Barbara.

The 90-minute debate unfolded scarcely a month before the election, the first in a series of high-stakes encounters between the president and his Democratic challenger. The two men meet Oct. 8 in St. Louis and again on Oct. 13 in Tempe, Ariz.

The polls gave Bush a slight advantage, with several key battleground states exceedingly close.

Bush and Kerry differed over North Korea, Iran and Russia as well as Iraq in a debate limited to foreign policy and terrorism.

“The only thing consistent about my opponent’s position is he’s been inconsistent,” said Bush, eager for the last word.