The Conservatives have taken a hit in the polls as the Liberal leadership race boosts that party and the New Democrats continue to retain strong national support. That the Tories are on a downward trend is undeniable, but their recent history suggests that they have been through this before.

The latest polls have been consistently bad for the governing Conservatives. The three most recent polls conducted over the last month put the Tories at between 29 and 32 per cent support, well below the 40 per cent they earned on election night in 2011. The New Democrats have been tightly packed at between 26 and 27 per cent, while the Liberals scored 25 per cent in the oldest of the three polls, and 29 and 30 per cent in the two most recent ones.

For the Conservatives, this is not a sudden dip. Speaking about his latest national survey earlier this month, Nik Nanos said that "polls are snapshots in time, but it's the trend that counts. And when you look at the trend line for the Conservatives…it's basically negative."

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By taking the snapshot element out of the equation entirely, and focusing just on the trends, we see that Mr. Nanos is absolutely correct – and not just in reference to his own polling. The trend line for the party, as shown in chart #1 which compiles every poll that has been released since the election, has been on a downward slope since 2011.

The negative trend has been relatively consistent, as the party was scoring nearer to 40 per cent shortly after its election victory, before sliding to around 30 per cent today. By comparison, the New Democrats have been holding steady. With the exception of the few months after Thomas Mulcair's leadership win that put the party in the mid-30s, the NDP has generally polled around, or perhaps slightly below, the 31 per cent that they got on election night.

The Liberals, on the other hand, have been less consistent. Their leadership race seems to have injected a bit of volatility to their polling numbers, but nevertheless the trend line is a positive one for the party. Indeed, their last set of polls has been among the best that they have had since the election.

The Conservatives have every reason to be concerned with this drop in support over the last 22 months. Sooner or later, a government loses favour. But this is not the first time that the Tories have experienced a mid-mandate decline.

As chart #2 shows, the Conservative trend line was also negative 22 months after the 2008 federal election (note the greater number of polls that were conducted between 2008 and 2010, when Stephen Harper presided over a minority government). The Tories experienced a brief spike in support in the wake of the coalition affair, before dropping below the Liberals when Michael Ignatieff became leader. They then experienced another spike in support before dropping to the low 30s.

The Liberals, however, were also experiencing a generally negative trend, while the New Democrats were slowly gaining instead of holding steady, as they have done since 2011. And that was in contrast to how their support moved in the 22 months following the 2006 election (see chart #3). Then, the Liberals were on a slight incline while the NDP was headed in the other direction.

But the Conservatives, again, were generally on the downswing. The negative trend was nowhere near as marked as it would be in 2010 and in 2013, but it was nevertheless heading south. The main difference was that the Conservatives were getting some of their better polling results at the 22-month mark in 2007 (though they were mixed – polls taken in the 22nd month put the Tories as high as 42 per cent and as low as 32 per cent).

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Chart #4 shows just how consistent this pattern of slow decline from an election victory has been for the Conservatives. The trend line for 2011-13 is very similar to that of 2006-07 and 2008-10, and of course the party was able to pull out of the dive and win in both 2008 and 2011. Can they do it a third time before the 2015 election?

The numbers suggest it could be the most difficult comeback to pull off. Though the slope is only marginally different, the trend line for 2011-13 is worse than that of 2006-07 or 2008-10. At this point in their mandate in 2007, the Conservatives were averaging 36 per cent in the polls. In 2010, that average was 33 per cent. In the last 30 days, the Conservatives have averaged 31 per cent support.

Though their slide in the polls almost two years into their mandate is nothing new, the situation the Conservatives find themselves in is unlike what they experienced in the past. In both 2007 and 2010, the Conservatives were consistently polling ahead of the Liberals with little overlap (as is clearly visible in charts #2 and #3). The New Democrats were firmly ensconced in third place. Now, the three parties are massed together and Mr. Harper is facing off against a tough opposition leader in Mr. Mulcair and, in Justin Trudeau, the Liberals are about to choose a wildcard leader. The trends may not reverse themselves this time.

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