MONTREAL—The advent of Stéphane Dion as the leader of the Liberal party of Canada presents Prime Minister Stephen Harper with both a huge opportunity and an immense risk.

It offers the Conservative leader his best shot ever at consolidating his own coalition. But what looks like a window of opportunity today could become a deadly trap for the Prime Minister if federalist fortunes take a turn for the worse in the next Quebec election.

At least in the short term, the outcome of the weekend's leadership convention diminishes the Liberal capacity to swiftly build a pan-Canadian progressive coalition to defeat Harper.

Outside Quebec, Dion is currently less attractive to New Democrat sympathizers than Bob Rae could be expected to be and in Quebec he is less likely to raise his party from the dead than Michael Ignatieff would have.

The new Liberal leader will need more than the few months that may be left before Canada takes a return trip to the polls to fix the latter.

This weekend, Dion was the second choice of enough delegates from the rest of Canada to vault from fourth place in the September race for delegates to a decisive fourth-ballot convention victory, but he was not even the last choice of the vast majority of the Quebec delegates.

Almost to a man and a woman, the party's senior organizers rallied to Ignatieff on the final ballot. That includes Martin Cauchon and Denis Coderre. For the two former ministers, Dion's victory is a double disappointment as it dashes their own plans to secure a headstart in a future run for the leadership.

For all the talk of a generational change, it is not strictly out of admiration or abnegation that the rising stars of the party outside Quebec rallied so readily to Dion on the convention floor. By ensuring the victory of the only Quebec candidate on the ballot, Martha Hall Findlay, Gerard Kennedy and others ensured that when the party next changes leader, candidates from the rest of Canada will be first in line.

Once Bloc Québécois strategists wipe the grin off their faces, they will notice that the outcome of the convention is a double-edged sword.

They had no way of knowing whether Ignatieff's success at building a loyal following among Quebec Liberals was a sign of things to come in an election or how Rae's progressive credentials would play with the many Bloc supporters who are alarmed by the current directions of the Harper government.

With Dion, they are in known territory, facing a familiar adversary that much of the francophone electorate has come to see in a terribly negative light over years of post-referendum skirmishes.

His victory may amount to handing Quebec to the Bloc and the Conservatives in the next election. But that would also mean that the Bloc could not count on a division in the federalist vote to sap Harper's strength.

Instead Tory incumbents would be able to fight the Bloc without having to worry about being outflanked by another federalist party and Conservative candidates would have a larger pool of voters to draw from in their battle for more Quebec ground.

Even as the Liberals were choosing their leader, Finance Minister Jim Flaherty was confirming that he expects to include a deal on the fiscal imbalance in his next budget. Solving the fiscal imbalance to the satisfaction of the provinces, in particular Quebec, is now more central than ever to Harper's strategy and to his hopes to neutralize his new Liberal opponent.

For if Dion stands for anything it is national unity and the environment.

In the latter case, his victory positions the Liberals on a crowded ice, where they will have to jostle with the NDP, the Green party and the Bloc Québécois for a position. Dion will also be weighted down by a Liberal record that is less than stellar.

Moreover, by putting so many eggs in the environment basket, the other parties may be tempting Harper to take a more agressive course on climate change. Having just witnessed his stunning moves on Quebec's national character and on the income trusts, does anyone doubt the capacity of this prime minister to change tack ?

Unity is another issue. As he watched Dion come from behind to win the leadership Saturday, Harper must have congratulated himself for having had the prescience to bring him in the loop of his plan to recognize that Quebecers form a nation. As a virtual co-author of the controversal motion adopted by the House of Commons last week, the new Liberal leader is in no postion to take advantage of what could be a major Conservative liability, at least outside Quebec.

But with Dion in the rink, Harper can also no longer assume that he owns the mantle of unity champion.

More than ever, the Prime Minister has a vital interest in helping Premier Jean Charest secure a second mandate in the provincial election that will be taking place over the next year, or, short of that, in going to the polls before his Quebec ally does.

For nothing might accelerate Dion's ascent in the rest of Canada, and little could do more to remobilize Quebec federalists behind the new Liberal leader, than a Parti Québécois victory and the prospect of another referendum.

At that point, many of the conditions for a replay of Joe Clark's 1980 demise at the hands of Pierre Trudeau would be in place.