Harper, who heads the largest opposition party, the right-wing Canadian Alliance, shouted the word at Chretien over the prime minister's refusal to back the US-led coalition against Iraq.
The House of Commons will debate Thursday a motion, moved by Harper, calling for an apology by the chamber to the United States "for offensive and inappropriate statements made against the United States of America by certain members of this House."
This is a reference to remarks attacking Washington made by ruling Liberal Party government ministers and backbenchers.
Minister for Natural Resources Herb Dhaliwal said US President George W. Bush had lacked statesmanship in the way he handled the international debate on Iraq; Carolyn Parrish, a Liberal backbencher, told reporters: "Damn Americans. I hate those bastards."
Harper's motion will also ask the House to "reaffirm that the United States to be Canada's closest friend and ally; hope that the US-led coalition in Iraq is successful in removing Saddam Hussein's regime from power; and urge the Government of Canada to assist the coalition in the reconstruction of Iraq."
Political insiders said the motion stands no chance of being passed as the government has a clear majority in the house, but that a watered down motion that praises the United States will likely be adopted.
Canada has said it would not support war in Iraq unless it was approved by the United Nations. But it has three ships, a handful of planes and some 1,000 troops, including integrated units, in the Gulf region.
Much of Wednesday's debate was over what might happen if 31 Canadian troops, serving in Iraq with British, American and Australian units on allied exchange programmes, are captured.
Harper and other opposition leaders wanted to know whether, under the Geneva Conventions, they would be treated as prisoners of war or "unlawful combattants."
Chretien and Defence Minister John McCallum refused to give a direct answer.
Harper, speaking to reporters, said Canada should be fully supporting the US-led war as well as backing Canadian and allied troops there.
"And then this clown (McCallum) gets up and makes these ridiculous defences about how you can have troops in the field and somehow they're not your responsibility and somehow you're not part of the war."
McCallum blasted the Canadian Alliance's remarks, saying in the House: "When they speak of us washing our hands of our troops or not supporting our troops I take extremely strong exception to that comment."