Liberal leadership contender Gerard Kennedy opposes a motion approved in the House of Commons, that recognizes that the Quebecois form a nation within a united Canada.

The motion was approved by the majority of MPs Monday night.

Kennedy argued it leaves open the possibility of advancing the separatist agenda.

The motion provides "official recognition to the idea of nation without defining it and that is irresponsible," Kennedy said at a press conference on Monday.

"It puts that official concept in the hands of the people who would use it for things that are frankly at odds with what most Canadians believe in," he said.

Kennedy said he felt obligated to voice his opinion.

"It's a wedge for future politics by Mr. Harper," Kennedy said hours before the vote, introduced by Prime Minister Stephen Harper last week, passed in the Commons.

"This is not a small thing -- this is about the identity of the country. It should not be played games with and I will not go along with that."

Part of the problem with Harper's "irresponsible" motion is that the notion of a "nation" is not defined, Kennedy said.

"How will it inform our challenge to bring this country together when there are four or five interpretations possible?" he said.

"This puts us into word-smithing, into semantics, and it puts us into games playing that I think has harmed this country in the past and should not be part of a future.''

The former Ontario education minister believes the motion raises expectations of the eventual constitutional entrenchment of Quebec nationhood.

"It's not good for Canada. It's a motion that puts a mark on the Constitution, whether it happens with the motion itself, (it) creates an expectation. It also changes the definition of the country," he said, appearing on CTV's Canada AM earlier on Monday.

"Canada is not some political chip to be played around with. Without any definition of what nation means, it will disappoint. It will create divisions that we don't have today," he said.

Kennedy said he decided to make his opposition public after considering how his position would affect his campaign.

"I don't know what the impact will be, but I know that I can't be a leader come December the 2nd or the 3rd and give a legitimacy to this idea moving forward," he said.

"I have an obligation, I believe, to my supporters to win but I have a bigger obligation to be the kind of leader that actually stands up for the things he believes in."

Liberals choose their new leader this week and Kennedy is the only leadership contender to reject the motion.

But the motion was supported by his seven rival candidates, as well as the Tories, most Liberal MPs, the NDP and even the separatist Bloc Quebecois.

"I wanted to make sure the Liberal party in future is not captured by the expediency that lays behind this motion,'' Kennedy said.

Kennedy's decision to buck the tide of political convention could give him a boost at this week's leadership convention as the voice of those opposed to recognizing Quebec nationhood.

But his main objective is to emerge as the leader of a unified party, he said.

"I have said we should not have this kind of debate in the middle of a leadership race. So what we need is a clear dialogue between the Liberal party and Quebec, and it's not possible to do so under those conditions," Kennedy said at Monday's press conference.

Kennedy is the only one of the four major contenders with virtually no delegate support in Quebec, where he won less than 2 per cent of the delegates elected two months ago.

But his stance could also reignite a divisive convention battle that the Liberals thought they had sidestepped with Harper's motion.

"This is a really difficult thing for the Liberals as they try to grapple with this issue because they've always been such strong federalists and now we have this issue of concept of a nation coming in. It's splitting the party," CTV's Ottawa Bureau Chief Robert Fife said.

Leadership front-runner Michael Ignatieff has enthusiastically welcomed Harper's motion, saying that that the push to recognize Quebec's nationhood began with his campaign.

His principal rival, Bob Rae, and the lone Quebec contender, Stephane Dion, have warily supported the motion despite reservations.

Ignatieff started the debate over Quebec's identity by coming out early in the campaign in favour of recognizing the province as a nation and eventually enshrining that status in the Constitution.

The Quebec wing of the party subsequently proposed a resolution, which will be considered at this week's convention, recognizing the province as a nation within Canada.

The resolution, which sparked a ferocious round of bickering within the party, also calls for creation of a task force to advise the next leader on the best way to "officialize" that status.

Ignatieff supported the resolution but his political foes, particularly Rae, Dion and Kennedy, opposed it, out of fears it would lead the country into another bout of constitutional wrangling.

Last week, the Bloc tried to drive the wedge deeper by introducing a motion calling on the Commons to recognize Quebecers as a nation -- with no mention of Canada.

Harper pre-empted the Bloc by introducing his own carefully worded counter-motion, specifying that the Quebecois form a nation "within a united Canada."