OTTAWA - Three federal byelections in early September promise to stir up Canada's stagnant political waters, serving as a litmus test for a possible general election this autumn.

Prime Minister Stephen Harper is expected to call the byelections this week in the Montreal-area ridings of Westmount-Ville Marie and St. Lambert and the southwestern Ontario riding of Guelph.

Opposition parties expect the vote in the three ridings will be held Sept. 8 but Conservative insiders suggest Harper may opt for Sept. 2, the earliest possible date.

The Westmount byelection must be called by Saturday under Elections Canada rules, but sources say the three vacancies will be bundled together.

The byelections will give all parties a chance to move the political dial, which has barely budged for the last two years.

Despite various apparent setbacks and triumphs for each, national public opinion polls suggest Harper's Conservatives remain virtually tied with Stephane Dion's Liberals, both with tepid levels of support well short of what would be needed to win a majority.

However, the stakes are highest for the Liberals, who won two of the three ridings -- Westmount and Guelph -- in the 2006 election. The Bloc Quebecois won St. Lambert.

Dion has been spending the summer trying to sell voters, already hard hit by record-high gas prices and a weakening economy, on the merits of his risky proposal to impose a carbon tax, offset by cuts in personal and corporate income tax. The byelections will test not just his sales job but his judgment in making the so-called green shift plan the centrepiece of the Liberal platform for the next election.

Moreover, they'll test whether Dion has managed to solidify his fragile grip on the reins of his party after suffering embarrassing losses in two previous sets of byelections.

"They're important, sure, and we're treating them accordingly," said Senator David Smith, co-chairman of the national Liberal campaign team.

While Dion has won kudos for having the courage of his environmental convictions, the substance of his green shift plan has received mixed reviews from provinces and pundits and has been relentlessly denounced by the Tories as nothing but a tax grab.

But Smith said he believes the plan will be "something of a plus" during the byelections, particularly in Quebec where "there's always been a higher degree of commitment" to tackling global warming.

However, some Liberals are privately concerned that the same people who ran last fall's doomed byelection campaign in the one-time Grit fortress of Outremont are running the show once again in Westmount, where former astronaut Marc Garneau is carrying the party banner.

Indeed, the NDP, which scored a stunning upset in Outremont, is hoping for an encore performance in Westmount.

Although Westmount has been an unshakable Liberal bastion for decades, deputy NDP leader and Outremont victor Thomas Mulcair said his party's polling suggests that Garneau enjoys a meagre four-point lead over NDP candidate Anne Legace Dowson, a well-known CBC radio personality.

"Anne's numbers are the same as mine were" at the start of the Outremont campaign, Mulcair said in an interview. "We think it's takeable."

Smith allowed that the NDP, which ran a distant third in the riding in 2006, will probably do somewhat better this time out. But he was skeptical that the NDP could pick up enough votes to overtake the Liberals, who won almost three times as many votes last time.

The NDP are also hoping another relatively high profile CBC radio personality, Tom King, can score an upset in Guelph, which the Liberals claimed with a reduced 5,000-vote margin in 2006.

By contrast, the Conservatives are downplaying their chances of winning any of the three ridings, while simultaneously upping the ante for Dion.

"They are all opposition-held ridings and anything less than two Liberal victories will be a huge defeat for Stephane Dion's Liberals," said Tory spokesman Ryan Sparrow.

Nevertheless, Conservatives privately reckon that Guelph is their best bet for a possible upset. It's the kind of riding, in the belt outside Toronto with a history of voting both Conservative and Liberal, that Harper believes they'll have to win if he ever hopes to form a majority government.

The Tories are running current city councillor, Gloria Kovach, against Liberal newcomer Frank Valeriote, a lawyer.

And Harper clearly has some hopes for St. Lambert. He showed up in the riding last month for St.-Jean-Baptiste Day celebrations.

Tory strategists contend the riding is essentially federalist, that the Bloc managed to take it in 2004 and 2006 only because of anger over the Liberal sponsorship scandal and the personal popularity of the Bloc's candidate, Maka Kotto.

With Kotto now gone and anger over sponsorship faded somewhat, the St. Lambert byelection may depend on whether federalist voters turn to the Tories or return to the Liberals.

Smith said he'll be interested to see whether the Quebec byelections are effected by the lingering scandal over former foreign affairs minister Maxime Bernier and his ex-girlfriend with ties to biker gangs.