A dozen majority governments have been elected since the end of the Second World War, and of those only three have fared worse in the polls at the two-year mark than the current Conservative government of Stephen Harper.

The Conservatives have averaged 29.7 per cent support across all polls taken between April and June, straddling the two-year anniversary of the government's majority victory of 2011. That represents a drop of 9.9 points since the vote, a larger-than-average decrease in support than other majority governments have experienced since the 1950s.

Most governments see a mid-term slump

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It is certainly true that most governments fall into a mid-term slump. Of the last 12 majority governments elected since Louis St. Laurent's 1949 victory, seven of them have seen their support drop by the two-year mark. However, five of them had their support increase, suggesting that it is far from a rule that the mid-point is always a low point for a sitting government.

On average, majority governments have lost 3.8 points from the previous election by mid-mandate. Some lost dramatically: the Progressive Conservatives under Brian Mulroney had slumped 29.2 points two years into their second term, and were handed the worst defeat in Canada's electoral history in 1993. But Mr. Mulroney's Tories also dropped 18.5 points at the midway mark of their first term, and went on to re-election in 1988 against Liberal leader John Turner.

Mr. Harper's Conservatives are tied for eighth on the list, though Pierre Trudeau's Liberals had more support in 1976 than the Conservatives currently have in 2013. Of the three Prime Ministers who had suffered worse decreases than Mr. Harper, two were subsequently defeated (the Tories in 1993, then under Kim Campbell, and the Liberals in 1984, then under John Turner – also a word of warning for those who might be thinking of replacing the sitting Tory leader).

Of the six other majority governments that were in a mid-term slump, only two of them were re-elected (Mr. Mulroney's in 1988 and John Diefenbaker's in 1962). But Diefenbaker was only handed a minority government. And St. Laurent in 1957 and Trudeau in 1979 were defeated despite winning more of the popular vote than the Tories, who won more seats. That puts Mr. Harper in the murky middle. He would much rather have gained support at the mid-point, which has so far been a strong predictor of re-election.

What we can learn from the slumps

These numbers give a hint at how mid-mandate polls are far from meaningless. In all cases since the 1950s, a government doing well in the polls two years into a majority mandate has been re-elected, while governments who are slumping in the polls have had a much more mixed record.

Nevertheless, there is a silver lining for Mr. Harper. In eight of the previous 11 majority governments, the trend at mid-point did not continue to the subsequent election. In other words, a government that was down in the polls two years into their mandate ended up performing better at the ballot box by the next election, while those who were up in the polls subsequently lost some support. This suggests there is good reason to expect the Conservatives to do better than 29.7 per cent in 2015's federal election.

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There have only been three cases, however, where a majority government increased its support in the next election compared to its standing at mid-mandate. It is an even smaller data set to work with, but on average growth from mid-point has been 6.8 points. That would put Mr. Harper's party at around 37 per cent, enough to be capable of re-election (though probably only with a minority).

But he has a long road back, and the most similar drops in support to Mr. Harper's (Pierre Trudeau's in 1976 and 1982) both led to defeat.

Might the current surge in support for the Liberal Party that is in part due to Justin Trudeau's arrival on the scene be distorting the numbers? That is a possibility, and there are few other cases of a new opposition party leader being named at around the same time as the two-year anniversary of a majority government. But there are two such examples: Trudeau/Clark in 1976 and Mulroney/Chrétien in 1990. In both cases, the newcomer went on to win the next election.

Éric Grenier writes about politics and polls at ThreeHundredEight.com .