The Liberals appear to be in big trouble in the Montreal riding of Outremont, where a byelection is to be held Monday, a poll suggests.

The Unimarket-La Presse poll published Friday suggests that NDP candidate Thomas Mulcair, a former Quebec Liberal environment minister, holds a six percentage-point lead over Liberal candidate Jocelyn Coulon.

"This could mean an historic breakthrough for the NDP if they could win the riding of Outremont," said CTV's Ottawa Bureau Chief Robert Fife. "It means they could possibly be a threat to the Liberals in Montreal but more importantly this could cripple Stephan Dion's leadership."

If Mulcair does win, he would be the first NDP MP from Quebec since consumer advocate Phil Edmonston won a byelection in 1990.

Liberals have held Outremont almost continuously since 1935, except when a Progressive Conservative won an election there for one term starting in 1988. Former Liberal MP Jean Lapierre won the riding by about 2,600 votes in the 2006 federal election.

The Liberals had been expected to hold Outremont. The possibility they might lose has the party hitting the emergency button, calling on Liberals in Ottawa to get to the riding this weekend to shore up support.

"A word of caution if I may -- the Liberals still think they might be able to win this riding, but only by about 700 votes," Fife told Â鶹´«Ã½.

The poll bodes ill for Liberal Leader Stephane Dion, he said.

"Mr. Dion is a Quebec leader. He was elected on the basis of being able to win seats in Quebec, and if he can't win Outremont, a traditionally Liberal riding, he is in trouble."

Supporters of former leadership rivals Michael Ignatieff and Bob Rae are already unhappy with Dion, he said, although a movement to ouster Dion before the next federal election is unlikely.

While there's been speculation about a fall federal election, Fife said that's unlikely, given these poll results.

In addition, the governing Conservatives may be poised to win in Roberval-Lac-Saint-Jean, formerly a Bloc Quebecois fortress, the poll suggests.

That riding is located in an area where the provincial Action democratique du Quebec made big gains in an election last March. The ADQ is philosophically similar in many ways to the federal Conservatives.

The Globe and Mail reported that the Bloc has called on its workers in eastern Quebec to join the fray in Roberval and help BQ candidate Celine Houde prevail over Tory Denis Lebel.

However, the poll suggests the Bloc should hold on to Saint-Hyacinthe-Bagot, the third federal Quebec riding up for grabs on Monday.

The poll suggests Prime Minister Stephen Harper's gamble of recognizing Quebec as a nation, has paid off, Fife said.

The Unimarket-La Presse poll conducted its surveys between Sep. 8 and 12. About 1,000 people were sampled in each riding, making for a margin of error of about three per cent.