The three main party leaders are in Quebec, as they prepare for the final leaders' debate.

NDP Leader Thomas Mulcair and Liberal Leader Justin Trudeau appeared at campaign events Thursday morning, while Conservative Leader Stephen Harper was in the province, but away from the hustings, a day before the French-language debate set for Friday.

For Mulcair, who saw his support in the polls slip after the French-language debate, the stakes are high. And that poses a major problem for the NDP leader, says CTV's Chief Political Correspondent Craig Oliver, as the party won its first-ever Official Opposition status in the 2011 election, due largely to its crushing win in Quebec.

"If this continues, his campaign will essentially be out of business," Oliver said of the NDP leader. "It's always a problem when you've got too many eggs in one basket, and his are all essentially in Montreal."

Yet, the drop in support for the New Democrats in Quebec has not resulted in a significant boost for the Liberals.

When asked why his party has not appeared to benefit from the NDP’s decline, Trudeau said he has been pleased by the response to Liberal candidates campaigning across the country.

The Liberal leader, who pledged Thursday morning to spend $20 billion for transit projects over the next decade, said his party is committed to investing in Canadians, including Quebec residents. Trudeau said, if elected, his party would help fund a light-rail line on Montreal's Champlain Bridge and a rapid transit project on the West Island.

"Quebecers are, like many Canadians, worried about their future," he said. "They need a political party that is willing to form a government that invests in them, and gives them that direct support."

Meanwhile, Mulcair said an NDP government would spend $200 million over four years to help retrofit homes and apartments across the country to help make them more energy efficient. The NDP leader said the country needs a new leader who has a long-term vision to fight climate change.

Oliver said while all of the party leaders will be looking to win seats in Quebec, the Liberals may have the most to gain if Mulcair continues to "leak support."

"If Mr. Mulcair is leaking support and on a downward spiral that we're beginning to see, it's hard to believe those votes are going to go to Harper," he said.

Conservative Leader Stephen Harper had no public appearances scheduled for Thursday, the day after he made a speech in Quebec City, where he laid out his party’s main priorities.