OTTAWA - Stephen Harper admitted Thursday that the U.S.-led invasion of Iraq was a mistake -- one that Canadian troops would have been plunged into had he been prime minister in 2003.

The grudging admission came during the second televised leaders debate as the five leaders discussed the Canadian mission in Afghanistan.

Bloc Quebecois Leader Gilles Duceppe needled Harper about the embarrassing revelation that whole sections of a speech about the Iraq war, delivered by Harper as opposition leader in 2003, were lifted almost word for word from a speech delivered two days earlier by Australia's prime minister at the time, John Howard.

A Conservative speechwriter resigned Tuesday after taking the blame for plagiarizing Howard's speech.

Duceppe said the Afghan mission has proved longer and more dangerous than anticipated because U.S. President George W. Bush diverted American troops to Iraq.

"If the situation is so tough in Afghanistan, certainly a large part of that is because of the error made by George Bush by going in Iraq," he said to Harper.

"Do you realize today that you were making a huge error by supporting Bush and Australia ... and would you make the same decision today as you were proposing Canada to do in 2003?"

The prime minister initially tried to dodge the question.

"I've made it very clear Canada is not going to Iraq. Obviously you know the answer to that question," he said.

But Duceppe continued to badger him as Harper tried to steer the discussion back to Afghanistan: "This is not the question I asked. . . I want to hear it. Do you admit it was an error of George Bush and you made the same error?"

Harper finally answered: "It was absolutely an error. It's obviously clear the evaluation of weapons of mass destruction proved not to be correct. That's absolutely true and that's why we're not sending anybody to Iraq."

Green Leader Elizabeth May shot back: "We're only not sending anyone to Iraq because you weren't prime minister at the time."

Harper's admission was in stark contrast to the speech he gave in the House of Commons -- much of it cribbed from Howard -- on the first full day of the Iraq invasion in March 2003.

"Alliances are a two-way process," he said at the time.

"We should not leave it to the United States to do all the heavy lifting just because it is the world's only superpower."

Harper urged Canada to join Bush's "coalition of the willing" in Iraq, relying heavily on American assertions that Iraqi strongman Saddam Hussein had stockpiled weapons of mass destruction.

"As the possession of weapons of mass destruction spreads, the danger of such weapons coming into the hands of terrorist groups will multiply, particularly given in this case the shameless association of Iraq with rogue non-state organizations," he argued.

"That is the ultimate nightmare which the world must take decisive and effective steps to prevent. Possession of chemical, biological or nuclear weapons by terrorists would constitute a direct, undeniable and lethal threat to the world, including to Canada and its people."

Harper was not so verbose on Thursday.

He had little to say beyond his admission that he and Bush were mistaken. Asked about it after the debate, Harper immediately changed the subject.