Why unreliable voters are key to Trudeau’s re-election hopes, and understanding the campaign ahead

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After they lost power federally in 2015, Conservatives liked to point out that they hadn’t lost that much support.

Despite an uninspired and poorly organized campaign on their part, their national vote total had only gone down by a couple hundred thousand. In a good number of seats they lost, they actually got more votes than in their previous, winning campaigns.

Their problem was that the electorate had dramatically expanded, far beyond the rate of population growth, with 2.9 million more ballots cast than in the election four years earlier.

That was because a great number of voters who often stay home had come out to cast ballots for Justin Trudeau’s Liberals. Combined with a collapse of the NDP that limited vote-splitting, the increased turnout propelled the Liberals to a majority government with 4.2 million more votes than in 2011.

Now, as the Conservatives seek to oust the Liberals after one term, there may be no bigger factor in either party’s fate than whether those millions of non-habitual voters − disproportionately younger Canadians excited by Mr. Trudeau in 2015 and potentially disillusioned now − come out again in October’s election.

And that segment of the electorate may be the single biggest key to understanding this year’s campaign − already evident in parties’ messaging, third-party efforts to help them out and a recent spat between the Tories and Elections Canada.

For the Liberals, the challenge is to somehow rekindle enthusiasm they generated as change agents − or at least a sense of high-enough stakes to stay engaged − while running as bruised incumbents.

They have shown in recent months how they will attempt to do that, to some extent, through policy promises geared toward millennials. They no longer have in their arsenal cannabis legalization (which they’ve acted on) and electoral reform (which they’ve abandoned). So they have to lean on economic, affordability and environmental issues, which members of Mr. Trudeau’s campaign team say were always more important to their younger supporters. Hence measures in this year’s budget − interest relief on student loans, mortgage support for some first-time homebuyers, student work placements − that they’ll try to highlight on the campaign trail. Likewise, commitments such as their recent pledge to ban single-use plastics, aimed largely at conveying planet-saving urgency to younger voters, even as some older ones might dismiss it as virtue-signalling.

They have also shown how much firing up supporters this time will involve replacing excitement around Mr. Trudeau with fear of the alternative.

When the Liberals sound alarms about Conservative Leader Andrew Scheer forming an unholy alliance with Ontario Premier Doug Ford, or darkly accuse Mr. Scheer of being a threat to abortion rights, it’s not primarily about persuading would-be Conservative supporters to turn away from him. More so, it’s about dissuading people who voted Liberal in 2015 from staying home (if they don’t splinter to the NDP or Greens) because they’ve concluded a Conservative government wouldn’t be that different from the status quo.

Conversely, the Conservatives have recently given indications that one of their main campaign aims is to not help the Liberals with that form of mobilization.

Mr. Scheer’s series of generally moderate policy speeches this spring, taking down the temperature on hot-button issues such as immigration, were partly aimed at expanding the Conservative tent – something he seems more interested in than did Stephen Harper, at least later on as prime minister. But a moderate tone in rolling out their agenda also has to do with avoiding sparking interest among people who would never vote for their party.

Some Conservative campaign-team members (although not all) also acknowledge that when their party is on the attack against Mr. Trudeau – as with its recent “not as advertised” commercials – the target audience is again not just people who might vote Conservative. At least as much, the ads seem to be about diminishing enthusiasm among centre-left voters. The Tories may be okay with those voters feeling a bit worse about their party for airing the ads, if the message leaves them feeling worse about their options in general.

The dynamic seems to be similar with the Conservatives’ third-party surrogates. Jeff Ballingall, who heads the Canada Proud operation that runs ads making fun of Mr. Trudeau, told The Globe his aim is to prove the Prime Minister is a “fake feminist, a fake environmentalist and a fake progressive,” and to show that while he “was cool in 2015, he’s not now.”

On the other side are third-party groups potentially helping the Liberals with turnout. Sometimes, as with the union-backed Engage Canada’s ads raising fears about Mr. Scheer, it’s relatively obvious. In others, as when environmental groups that support carbon pricing simply encourage participation in the election, it may not even be entirely intentional.

On balance, increased engagement of any voters – and again, especially younger ones – stands to benefit the Liberals more than the Conservatives, whose older support base votes more reliably than those of other parties. That helps explain why the Tories responded so angrily, last month, to a (subsequently scrapped) enlistment by Elections Canada of social-media “influencers” to encourage their followers to exercise their democratic right: The campaign seemed designed to encourage the sorts of otherwise unengaged voters who came out in 2015 to do so again.

Just how much voting is likely to drop off among such people in 2019 is very difficult to measure in advance. As veteran pollster Greg Lyle pointed out recently when asked about his sense of turnout intentions, people not disposed toward voting tend not to participate in lengthy surveys about voting intentions.

But plainly, the parties know how much of a variable those would-be voters add up to.

There are other segments of the electorate that are also very much targets. Hundreds of thousands swing between the Liberals and Conservatives and a higher number between the Liberals and parties to their left. But as the past election showed, the most important swing voters might be those who swing between voting Liberal and not voting at all.