In an ideal world, each of the 308 ridings in Canada would be of equal importance to the parties trying to form the next federal government – but they’re not. Some are so solidly supporting one party or another that, barring a major catastrophe between now and election day, the results are pretty much a foregone conclusion and the trailing parties are running candidates simply to “fly the party colours.”

Election campaigns are the epitome of strategy, and it makes no strategic sense to expend valuable and limited resources – be it time or money – in places where you’re almost certain to win or almost certain to lose. This is why so much of the 2011 federal election is being played out in a relatively small part of Canada – the swing ridings.

There are several ways to define a swing riding, but one of the most common is to look at margins of victory in the last election. There are 33 ridings (representing just under 11 per cent of the total) where the 2008 election was decided by a margin of five per cent or less. Of these, nine were won by a margin smaller than one per cent, nine by a margin of between one and three per cent, and 15 by a margin of between three and five per cent.

Swing ridings are scattered across Canada, in every province and in two of the three territories. But a significant number of them are concentrated in three main areas: Vancouver and the Lower B.C. mainland, the Greater Toronto Area (including Kitchener, Guelph and Niagara) and some ridings in Quebec. And that helps to explain why the three main national party leaders are spending so much of their time campaigning in a relatively small number of places. (It also helps to explain why both Conservative leader Stephen Harper and Liberal leader Michael Ignatieff were both in Yellowknife Sunday night, battling for the hearts and minds – and votes – of electors in the riding of Western Arctic, with Harper trying to win the seat from the New Democrats and Ignatieff trying to win over enough voters to prevent that from happening. In this election, one seat won by the Conservatives could mean the difference between a majority government and a minority one.)

The closeness of the vote in swing ridings means national opinion polls don’t necessarily give an accurate indication of what the final seat tally will be on May 2. When a riding can be decided by just 17 votes (as Ontario’s Kitchener-Waterloo was in 2008) or 20 votes (Vancouver South), national polling numbers showing the Conservatives with a comfortable lead over the Liberals are irrelevant. Nor do publicly available opinion polls within these critical swing ridings offer much insight, as they have such a small sampling size that the margin of error is too big to give an accurate idea of which candidate is likely to win. (The parties will have more accurate poll numbers in each of these swing ridings, but they’re not sharing those results with the other parties or the public at large or through the media. The best indication of how comfortable they feel about the result in a particular riding will be how often a party leader visits in the next two weeks and how much advertising they pour into the riding.)

Swing ridings could have another important effect on this election: giving people in British Columbia the deciding votes on whether the next government will have a majority or minority, with 10 of the 36 B.C. ridings considered swing ones. It’s unlikely any party will win all 10, but if the Conservatives are able to pick up a few seats in Atlantic Canada and Ontario, and hold on to most of their seats in Quebec, a few more swing seats in B.C. could be enough to give them the majority that Stephen Harper has been asking for. The Liberals and the NDP, meanwhile, need to pick up enough of the swing ridings across the country to keep that from happening.

It’s hard to predict which swing ridings will be the key ones to watch, but here are a few where things could get interesting – and why you can expect to see a lot more of the party leaders in these places between now and May 2 :

Atlantic Canada: Egmont, Fredericton, Moncton-Riverview-Dieppe, Saint John, St. John’s South-Mount Pearl

Quebec: Beauport-Limoilou, Charlesbourg–Haute-Saint-Charles, Gatineau, Jonquière-Alma

Ontario: Brampton-Springdale; Brampton West; Guelph, Kitchener Centre, Kitchener-Waterloo, Mississauga-Erindale, Welland, York Centre

Western Canada: Edmonton-Strathcona, Saskatoon-Rosetown-Biggar, Winnipeg North

British Columbia: Esquimalt-Juan de Fuca, Vancouver Island North, Vancouver South

This post was originally written by Graham Green, who is no longer working at Hill+Knowlton.