Prime Minister Stephen Harper has called three federal byelections for Sept. 8, setting the stage for a possible general election later this fall.

The results of the Sept. 8 vote will reflect the impact of the slowing economy on Harper's Conservative party fortunes. They will also provide an early indicator of any bounce or bottoming out as a result of Liberal Leader Stephane Dion's decision to make the environment the centrepiece of his summer stumping at a time when manufacturing jobs are disappearing in Canada's two most populous provinces.

The Conservatives are actively minimizing their chances in the byelections, belying significant efforts they're making in at least two of the contests.

The Liberals held two of the three ridings, and a failure to hold those two seats would undoubtedly raise questions about Dion's leadership.

Two vacant Quebec ridings, Westmount-Ville Marie in Montreal and nearby Saint-Lambert, along with the Ontario riding of Guelph are up for grabs.

The Liberals held Westmount and Guelph, and the Bloc Québécois first won the longtime federalist riding of Saint-Lambert in 2004.

Dion, whose party holds the balance of power in the minority Parliament, said this week he now believes Canadians are ready for a fall campaign.

"We have seen over the winter and the spring more and more interest for federal politics," Dion said earlier this week. "And more and more appetite for an election."

But the Liberals have gone to great lengths to avoid bringing down the Conservative government during multiple confidence votes over the past year.

Byelections litmus test for Liberals

Party insiders predict the outcome of the newly announced byelections could either embolden the Liberals or deter them from making good on Dion's autumn election musings.

In Guelph, the NDP is hoping a relatively high profile CBC radio personality, Tom King, can score an upset. The riding has a history of flipping back and forth between the Grits and Tories, having been held by the Conservatives when former prime minister Brian Mulroney was in office.

Liberal MP Brenda Chamberlain's retirement puts it in play again.

This riding is a mix of rural and suburban voters — the kind of turf the Conservatives successfully targeted in the 2006 campaign, but it also includes workers who have lost jobs because of Ontario's sputtering manufacturing sector. It will be one of the most closely watched races of the evening.

Chamberlain claimed the Guelph riding in the last election by a 5,000-vote margin.

NDP goes for 2 in Quebec

Moments after the prime minister announced the byelection dates, NDP Leader Jack Layton scheduled a Monday news conference to introduce his party's slate of candidates for the three ridings.

Westmount has been a Liberal fortress for decades. Dion has pegged former astronaut Marc Garneau as the party's standard-bearer there, even though Garneau was defeated in another riding in the last general election.

But another Montreal Liberal enclave – Outremont – was supposed to be a sure thing for the Liberals during a byelection last September and the New Democrats easily out-distanced the Liberals to send Thomas Mulcair to Ottawa.

The NDP is hoping to enjoy a repeat of their upset by fielding CBC Radio host Anne Lagacé Dowson as their Westmount candidate.

Montreal labour lawyer Guy Dufort is running for the Tories in Westmount. The Bloc is fielding Charles Larivée, and Claude Genest is representing the Green Party.

Bloc will face off with Tories on South Shore

Saint-Lambert will be an interesting test of Bloc Québécois strength. The riding was staunchly federalist and Liberal for decades. Disgust with the Liberals over the sponsorship scandal helped hand it to the Bloc in 2004 and 2006.

The popular Bloc MP Maka Kotto has stepped down and gone to the provincial Parti Québécois. The Conservatives covet this riding as a signal they've become the de facto federalist alternative to the Bloc.

It was no accident that Harper spent Quebec's Fête Nationale in this riding across the St. Lawrence River from downtown Montreal.

Dion in the hot seat

Dion's leadership, already being questioned by some in his party, would come under tremendous scrutiny should the Liberals be defeated Sept. 8, on the heels of embarrassing losses in two previous sets of byelections.

The Liberal leader has been spending the summer fighting Conservative attack ads over his party's high-stakes plan to reduce Canada's greenhouse gas emissions by imposing a carbon tax, offset by cuts in personal and corporate income taxes.

The Conservatives maintain that anything other than the status quo in the three vacant seats will be a huge defeat for Dion.

"Anything short of two Liberal wins and one Bloc win with margins equal to or greater than 2006 will be failures for both Stéphane Dion and Gilles Duceppe," Conservative party spokesman Ryan Sparrow said in an e-mail.

Campaign issues spread far and wide

The Tories have already revved up their platforms for the byelections. The key planks are family tax cuts, tackling youth crime and strengthening consumer laws to protect the health of Canadians, Sparrow said.

Dion called on voters in the three ridings to "send Prime Minister Stephen Harper a strong message that they don't like the way he is running the country."

"We have an impressive group of candidates and, while we take nothing for granted, we are confident that voters in these three ridings will turn to the Liberal Party as the best option for a richer, fairer, greener Canada," Dion said.

For the Liberals, the environment and jobs are at the top of their campaign concerns.

"Guelph needs investment in innovation in order to attract more jobs," said Liberal candidate Frank Veleriote.

"We need meaningful action on the environment. We need to ensure that our universal health-care system stays that way. We need to strengthen our support for new Canadians, seniors, and veterans."

Recent national public opinion polls suggest support for both the Conservatives and Liberals has barely budged over the past few months, with the two parties virtually tied at support levels well below what's needed to win a majority government.