MONTREAL—If every party that entered a pre-election summer with an edge on the competition had beat its rivals to the finish line, Ed Broadbent in 1988, Kim Campbell in 1993 and Thomas Mulcair in 2015 would have each been elected prime minister.

Not only have campaigns been shown to matter but public opinion often gets reshaped by events beyond the parties’ control along the way.

Syrian child refugee Alan Kurdi’s drowning at the time of the last campaign was a case in point. More than a few voters found the Conservative reaction short on empathy. And that played to a perceived weakness of the Stephen Harper team.

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The best-laid plans of Canada’s federal parties on the way to the Oct. 21 vote are no less at risk of unforeseen change in the dynamics.

Take Justin Trudeau’s announcement this week that his government is going ahead with the expansion of the Trans Mountain pipeline. Polls suggest a majority of Canadians find the decision reasonable. But it might only take a major oil spill to change the optics on the prime minister’s call.

And what of federal Conservative Leader Andrew Scheer’s long-awaited climate policy? It was unveiled on a nice sunny day against the pristine backdrop of a western Quebec pond. Conservative strategists have to thank their lucky stars for that clement weather. The area has been prone to tornadoes over the past few summers.

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Andrew Scheer delivers climate plan with no targets

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But if the Conservative party does not want the many holes that are being poked by climate policy wonks in his environmental platform to register increasingly loudly with voters, it has to hope for a summer and a fall similarly free of severe climate events.

When it comes to climate change and pipelines, Canada’s main party leaders are not only at the mercy of so-called acts of God.

Six months ago, few political insiders factored the Green party in their election calculations. That changed as Elizabeth May’s party and the Green brand became steadily more popular. One recent poll even had her party ahead of the NDP nationally.

As her party eats away at NDP but also Liberal support, May has become a threat to Trudeau’s re-election prospects. But Scheer also has to do his part to keep things that way.

His first strategic mission at the time of his climate announcement on Wednesday was to appease the conscience of the swing voters whose priority is getting rid of the Liberals in the fall by showing them an environmental plan.

But by now Scheer also had to play to a secondary audience made up of voters committed enough to a more proactive climate agenda that they are considering a switch from the Liberals to the Greens.

Ideally the Conservative party would like to convince those voters that the climate change shortcomings of a Scheer government would not really be that much worse than those of the ruling Liberals. In that spirit, a significant part of the Conservative document was devoted to badmouthing Trudeau’s policies.

Political developments at the provincial level also often play out in unexpected ways. And no, this is not just about how Ontario Premier Doug Ford is turning into Scheer’s albatross.

By appealing to other Canadians’ solidarity with Albertans over the course of the Trans Mountain travails, both former premier Rachel Notley and Conservative successor Jason Kenney likely made the audience for Trudeau’s pipeline decision more receptive.

Whether any of those developments will still be top-of-mind next fall could depend on a series of unknowns.

Subject to developments in the U.S. Congress for instance, Parliament could be recalled later this summer to ratify the new North American trade deal. That would provide the Liberals with an opportunity to showcase what many independent critics consider to be their most adult foreign policy initiative.

And then there is no fixed timeline on the ethics investigation on SNC-Lavalin. The Liberals would be happy not to hear from commissioner Mario Dion until after the election. Prudence would dictate he not drop his report in the middle of the campaign but sitting on his findings — if they are ready for publication — until Canadians have voted is not a more palatable alternative.

Earlier this year, the Ontario Court of Appeal heard the Ford government’s challenge to Trudeau’s carbon tax. It could issue a ruling before the election.

Saskatchewan’s top court has already upheld the measure. If the Ontario court issued a decision along the same line, Trudeau’s carbon tax legal case would be greatly reinforced.

But would a second court victory necessarily be a blessing for a campaigning Trudeau or would it simply convince more voters that the only way to get rid of the carbon tax is to vote for the Conservative party? Stay tuned — this election has not really begun to play out!