There is no doubt that the federal parties are playing politics with the election debates. And why wouldn't they? The stakes are tremendously high. No other event has the potential to have such a major impact on an election as a leaders' debate.

Polling during each of the last three federal election campaigns gives clues as to the kind of impact the next one might have.

Correlation does not equate to causation, of course, and it is impossible to know for sure what impact a leaders' debate has had on the polls in the past. Other events may have played as great or an even greater role.

But debates are watched by millions of Canadians and receive an enormous amount of coverage in the media. Not only can the debates form an important impression on viewers, they can also shift how a campaign is covered. While changes in voting intentions around a debate may not be entirely related to the debate itself, these direct and indirect effects are undeniably important.

Over the last three elections, debates have moved the polls (considered here as the average of polls taken in the seven days before and after a debate) by an average of plus or minus 1.4 percentage points per major party. The French-language debates have had a larger impact, shifting the polls by an average of plus or minus 3 points.

But these numbers mask individual cases where dramatic movement has taken place.

2005-06: Harper vs. Martin

For Stephen Harper, the only leader of the three main parties who won't be taking part in his first leaders' debate this fall, debates have played an important role in his electoral fortunes.

Conservative Leader Stephen Harper strengthened his position over Liberal Leader Paul Martin following the French-language leaders' debate Jan. 10, 2006 in Montreal. (Tom Hanson/Canadian Press)

The leaders' debates in December, 2005 did not change the dial very much in either direction for any party. By the January, 2006 debate, however, the Conservatives were on the upswing. From an average of 30 per cent support around the December debate, Harper began the January debate with 35 per cent support in polls over the previous week. In the week following the debate, his support jumped to 38 per cent as the Liberals under Paul Martin tumbled from 31 per cent support to just 28 per cent.

The French-language debate was even more important for Harper. Conservative support in Quebec increased from 18 per cent prior to the January, 2006 debate to 27 per cent in the week after it. Both the Liberals and the Bloc Québécois suffered as a result.

2008: Harper vs. Dion

While the 2006 debates helped propel Stephen Harper's party to victory, the 2008 debates may have prevented him from winning a majority.

Prior to that year's debates, the Conservatives were polling at 37 per cent nationwide, holding a 12-point advantage over Stéphane Dion's Liberals. A majority government loomed. But the debates went well for Dion. Though debate-night polling did not choose him as the winner of the English-language debate, he did improve people's opinion of him, whereas Harper worsened that opinion. And Dion was seen as the winner of the French-language debate.

The polls reflected this. The Conservatives dropped to 34 per cent in post-debate polling as the Liberals inched up to 26 per cent. In Quebec, however, the Conservatives fell from an average of 25 per cent to just 19 per cent, as the Liberals jumped from 19 per cent to 24 per cent.

2011: Layton vs. Harper, Ignatieff and Duceppe

The Liberals had no such luck in the 2011 debates, which marked the beginning of the party's decline in that campaign. It was not that leader Michael Ignatieff had done very poorly — roughly as many people selected him as the debate winner as they did Jack Layton in both languages — but rather that Layton left people with the most positive impression.

Going into the mid-April debates, the Conservatives were on track for a majority government with an average of 40 per cent support in the polls. The Liberals were trailing with 28 per cent, while the NDP was at 18 per cent. In Quebec, the NDP was showing a little life, having improved from 15 per cent to 18 per cent since the start of the campaign, but the Bloc under Gilles Duceppe still looked dominant at 34 per cent.

In the week following the French-language debate, which after viewing many more people told pollsters that their opinion of Layton had improved than any other leader, the New Democrats surged to 25 per cent support in Quebec, with the other three major parties falling back. From a slow gain of three points in the province over the campaign's first two weeks, the NDP would start picking up one or two percentage points per day and all but destroy the Bloc Québécois.

The NDP did not experience as much of a gain nationally after the English-language debate, but was nevertheless up three points in the following week. Ontario would eventually get on the bandwagon later in the campaign and help propel the NDP past the Liberals.

None of these debates may have had the fabled "knock-out punch," but their impact was nonetheless significant. In 2006, Stephen Harper demonstrated he could be prime ministerial. In 2008, Stéphane Dion turned around some of the bad impressions he was leaving in voters' minds. And in 2011, Jack Layton finally convinced many Canadians his party was a viable alternative.

Debates can go a long way towards confirming voters' fears and concerns, or dispelling them. The polls now show all three parties in a tight race. No wonder the debate over the debates has become a political battlefield.

This article reviews trends in public opinion surveys. Methodology, sample size and margin of error if one can be stated vary from survey to survey and have not been individually verified by the CBC.