When Quebec separatists stumble and provide a political gift to the federalist cause, it’s part of the Prime Minister’s job description to make the most of it — not for himself, but for Canada.

So when Bloc Québécois MP Carole Lavalée put her foot in it this week by accusing Ottawa of mounting a federalist propaganda campaign with invitations to students to visit Canada’s national parks for free, Stephen Harper should have been able to hit a home run for the cause of national unity.

Jean Chrétien, Brian Mulroney and Pierre Trudeau would have stepped up to the plate, regardless of their partisan stripes. Separatism may be quiescent in Quebec these days, but this is also a time of political tumult in the province, where the federalist banner is in retreat because of recent stumbles by Liberal Premier Jean Charest. The Parti Québécois seems poised to regain power after the next provincial election.

Against that backdrop, any Canadian PM would move in for the kill and belittle the Bloc for displaying such paranoia over the attraction that Banff’s rugged beauty might hold for the malleable minds of young Quebecers seduced by free admission into national parks.

Harper’s killer instinct, however, is more partisan than patriotic. To be sure, he beat up on the Bloc — but only enough to make the larger, improbable point that Canada’s opposition parties can never be trusted with national unity. The Liberals and New Democrats, Harper argued, have been and always will be in bed with the Bloc and should thus be disqualified from ever wielding power.

The PM seems unable to resist dredging up the abortive Liberal-NDP coalition plan of late 2008, in which the BQ merely agreed not to vote to bring down the government for a specified period — even though Harper had sought a similar understanding when he was opposition leader.

Apart from the partisan pettiness, this is the equivalent of scoring an own goal in soccer. Rather than trying to drag down the opposition, Harper should be using this BQ gift to talk up Canada in Quebec.