OTTAWA (Reuters) - In the final week of Canada’s election campaign, the leaders of the opposition Liberals and New Democratic Party are locked in a fierce and divisive battle for the same center-left votes.

Liberal leader Justin Trudeau greets supporters during a campaign stop in St. Catharines, Ontario October 14, 2015. REUTERS/Chris Wattie

Behind the scenes, however, insiders from both parties tell Reuters there will be huge pressure on the leaders to forge some sort of a power-sharing agreement after the Oct. 19 vote rather than risk a deadlock and another election.

The willingness of Liberal chief Justin Trudeau and NDP boss Thomas Mulcair to seal a deal could have a direct bearing on whether Canadians have a stable government after polls close on Oct. 19, or face a prolonged period of political instability.

Polls show no party is likely to win a majority of seats in the House of Commons, creating an opportunity for the Liberals and NDP to defeat Prime Minister Stephen Harper’s ruling Conservatives in a confidence vote.

No one pretends the two opposition leaders are close. Mulcair, 60, refers dismissively to his younger rival and calls him a closet Conservative on major issues.

Trudeau, 43, citing personal differences and what he describes as Mulcair’s bad decisions on taxes and investments, says he cannot work with his counterpart.

But senior Liberal party figures will make it clear to Trudeau that he should resolve his issues rather than do anything that might keep Harper in power or give him a chance to win it back, three well-placed Liberals said.

Separately, one high-ranking New Democrat predicted an “irresistible force coming from supporters of both parties” on the two sides to work together if the circumstances were right.

“The imperative would remain quite high despite things that were said during the heat of battle,” said the New Democrat.

The Liberals dismiss the idea of a formal coalition where both parties have ministers in a joint government. More likely would be a type of agreement whereby the junior party agrees to allow the larger party to rule in return for specific concessions.

If Trudeau and Mulcair fail to work out such a deal, whoever was the prime minister would have to govern day by day, knowing he could be brought down at any time. That could spark a new election, which would give the Conservatives a chance to regain power.

“It’s going to have to work to some degree ... another election could lead to the same place,” said a fourth Liberal source.

HURT FEELINGS

Harper led two successive minority governments from 2006 to 2011, relying partly on political skill and a divided opposition to stay in office. But his chances of keeping another minority alive after Oct. 19 appear slim given the determination of the opposition to bring him down.

Although the two left-leaning rivals have worked together in the past, mutual mistrust can run deep.

Asked on Wednesday about cooperation with Mulcair, Trudeau declined to answer directly, saying Canadians did not “want politicians organizing back rooms deals around who actually gets to wield power.”

His dismissive tone has hurt Mulcair, who says he is prepared to work with the Liberals. The NDP platform, released earlier this month, goes even further, pledging to “work with other federalist parties through informal or appropriate stable arrangements to end Stephen Harper’s lost decade.”

Mulcair complained on Tuesday that “every time I’ve opened that door ... it’s Mr Trudeau who takes it upon himself to slam that door shut.”

The Liberals date back to 1861 and have ruled Canada longer than any other party, while the New Democrats were created in 1961 and have never held power federally.

The New Democrats kept the Liberals in power from 1972 to 1974 with Trudeau’s father, Pierre, as the prime minister. But the NDP lost half their seats in a subsequent 1974 election.

The two parties also agreed to create a coalition in late 2008 to bring down Harper’s minority government, but the Liberals walked away in the aftermath of a leadership crisis.

Former New Democrat legislator Dawn Black, who took part in the 2008 talks, said “the Liberals had an entitlement kind of attitude” that made negotiations hard.

“We still see it in Trudeau daily ... and I don’t think that will change very easily,” she said.

Martha Hall Findlay, a Liberal legislator at the time, said “the problem facing the current folk is that the history of what happened in 2008 has made it very difficult ... to do a repeat.”

Yet while the memory of the failed coalition still stings, some circumstances have changed.

Trudeau has shifted his party leftwards into New Democrat territory, talking of taxing the nation’s wealthiest, the need for social responsibility and more spending. At the same time, the New Democrats moved to the right, stressing the importance of balanced budgets.

Former senior Liberal official David Zussman said he did not think the parties’ ideological differences would be a major problem.

“The bigger fear I think would be the smaller party wondering whether that was the end of them,” he said, citing the often sorry fate of junior partners in governments.