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OTTAWA -- Prime Minister Stephen Harper is embarking on a combat mission in Iraq with no political cover should anything go awry.

New Democrats and Liberals refused Friday to support Harper's decision to join in airstrikes against the extremist Islamic State of Iraq and the Levant (ISIL), which has been committing atrocities in northern Iraq.

Harper appeared to anticipate going it alone, telling the House of Commons: "I do this recognizing that in a democracy, especially one approaching an election, there is rarely political upside in supporting any kind of military action and little risk in opposing it."

The risk for Harper comes not at the outset of the mission. Indeed, one public opinion poll has pegged initial support for airstrikes at almost two-thirds of Canadians.

However, as Liberal and Conservative governments discovered during the 12-year combat mission in Afghanistan, public opinion can sour quickly if Canadian lives are lost or a conflict is perceived to be dragging on without any obvious success.

For opposition parties, the risk is that the mission does, in fact, turn out to be a quick in-and-out affair that leaves ISIL in tatters. They're betting the chances of that are near zero.

Opposition parties are convinced Harper can't stick to his promise that the mission will be "for a period of up to six months" and won't be allowed to turn into "a prolonged quagmire." The promise rang all the more hollow to opposition MPs when, in the next breath, Harper revealed the airstrikes could be expanded to include bombing ISIL insurgents in Syria.

"From mission creep to mission leap," quipped NDP Leader Tom Mulcair.

Mulcair noted that the U.S. has been trying unsuccessfully to defeat Islamic extremists in Iraq, "under one name or another," since its "wrong-headed" invasion in 2003.

"The prime minister insists that this mission in Iraq will not be allowed to become a quagmire but is that not precisely what our American allies have been facing in Iraq for the last 10 years? Will Canada be stuck a decade from now mired in a war we wisely avoided entering a decade ago?"

Mulcair had been clear from the outset that the NDP was highly unlikely to support any combat mission. New Democrats had refused to support even a non-combat role for several dozen special forces members sent to help advise Iraqi forces for a 30-day period.

Justin Trudeau's 36 Liberal MPs had been the likeliest source of political cover for Harper, having backed the non-combat mission.

But Liberal sympathies shifted subtly last week after former Liberal prime minister Jean Chretien warned that involving Canadian forces in Iraq, even in a non-combat role, could suck Canada into a decades-long conflict.

Chretien's popularity soared when he chose to keep Canada out of the 2003 Iraq war.

Liberal sympathies shifted even more noticeably after Harper used a New York audience to reveal that the United States had asked Canada to make a greater, combat-specific contribution to the fight against ISIL.

Having backed the non-combat role, insiders say Liberals were surprised that Harper made no effort to brief them on the U.S. request or to make the case for joining other nations in conducting airstrikes. The secrecy left Liberals distrustful of Harper's motives and fearful that there really was no coherent plan to eradicate ISIL, other than to be seen to be doing something.

Trudeau signalled his intention to oppose a combat mission in a speech to a think-tank conference on Thursday. He repeated big chunks of that speech in the Commons on Friday.

"We know there is a role for Canada to be involved in the fight against ISIL but there is a clear line between non-combat and combat roles," he said.

"It is much easier to cross that line than to cross back. It is always easier to get into a war than to get out of one."

In the past, Liberals have frequently been torn over Canada's role in combat missions, splitting into hawks and doves over Afghanistan, for instance. But this time, insiders insist distrust of Harper left the caucus united in its opposition to a combat role in Iraq.

Green party Leader Elizabeth May also voiced her opposition to a combat mission, arguing that provoking a military response is a precisely what Islamic extremists want to fuel hatred for western nations.

"What kind of barbaric terrorist organization beheads westerners and puts it on YouTube unless their exact goal is to get us to do what we are now doing and engage militarily in the region?"