In the hours following Ontario's last election, as a Toronto news channel cut to commercial, there flashed some footage of what appeared to be the evening's best victory party.

It was not Kathleen Wynne's festivities, or those of any other Liberals celebrating their party's return to majority government. Instead, while most New Democrats around the province were grumbling about their failure to make electoral gains, the happy scene was unfolding in the headquarters of re-elected NDP MPP Jagmeet Singh, where an energized dance floor of South Asian twentysomethings and teenagers was dancing the night away.

Looking like something even non-partisans might enjoy – accessible, fun, maybe even kind of cool – it was visual evidence of how Mr. Singh has offered hope to his party at a time when it has had few other bright spots east of Alberta.

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If Justin Trudeau and his campaign team watched the clip now, they would have good reason to feel a bit unsettled. Because on Monday, unless he changes his plans last minute, Mr. Singh will announce his campaign to become the leader of the federal New Democrats.

He may not get as much attention as he should, less than two weeks before the end of a Conservative leadership campaign that has thus far overshadowed the New Democrats' more nascent race. But there is good reason to believe that the dapper and charismatic 38-year-old former criminal-defence lawyer, now the Ontario NDP's deputy leader, poses more potential threat to Mr. Trudeau's Liberals than anyone seeking to lead either of the major opposition parties.

To some extent, any NDP leader might be more threatening to Mr. Trudeau than whoever winds up leading the Conservatives. That's not a reflection just on the Tories' contest, uninspiring though it may be, but also on long-standing voter patterns. Far more voters move between the Liberals and New Democrats than between either of those parties and the Tories, who have the most loyal backers and the most people unwilling to vote for them.

And if the Jack Layton era demonstrated anything, it's that when the NDP seems more in touch than the Liberals with blue-collar and middle-class voters, huge shifts can happen in a hurry.

New Democrats may well decide, before their fall leadership vote, that someone other than Mr. Singh – perhaps Charlie Angus, the colourful Northern Ontario MP with the punk-music past who has the most buzz of three candidates already in the race – is best suited to engineering such a shift. Maybe the scrutiny afforded to national leadership aspirants, rather than provincial third-party legislators, will expose a lack of gravitas or principle or discipline that disqualifies Mr. Singh.

But to have watched him, in the early stages of his political career, is to understand why his upside could prove irresistible to members of a party who have been waiting since Mr. Layton's death for someone to take it by storm.

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His most obvious offer, likely to get the most media attention as he launches, is being the rare politician – and very rare New Democrat – potentially able to beat Mr. Trudeau at his glamorous image game. Generational change?

He's younger than the PM was when he became Liberal Leader. Emblematic of a new, cool Canada? The first Ontario MPP to wear a turban is also the first to have his taste in tailored suits, bespoke bicycles and sports cars rewarded with a GQ feature. Tough enough to have beaten up a black-belt senator in a boxing ring? Mr. Singh was a seriously competitive mixed-martial-arts fighter.

But there is something no less valuable, and considerably more substantive, that Mr. Singh may bring to the table. Other than for a brief period under Mr. Layton, the federal NDP has struggled mightily to connect with working- and middle-class immigrants and visible minorities in particular – leaving the Liberals facing little competition to their left in suburban battlegrounds around the country's biggest cities. Mr. Singh has displayed a knack for championing policies that both appeal to those voters and adhere to the values-driven orthodoxy that mobilizes the NDP's base.

As an MPP, his push for stronger protections for temporary workers would certainly fit that bill. So would his activism, more than any member of the provincial legislature, against carding by police. Leading the charge for lower auto insurance rates, after he first won office in 2011, may have seemed like simple retail politics, but it addressed the perceived injustice of residents of Brampton (the suburb that he represents) paying especially exorbitant premiums.

Mr. Singh's pet causes to date do not add up to a national vision. But if he ever comes up with one, it could be as grounded in the struggles of marginalized people – those working hard to join the middle class, as Mr. Trudeau would put it – as anything else the NDP has put forward.

And then, tied in to both his charisma and his policy priorities, there is his ability to rally people around or behind him – in particular to convince young members of his community there's excitement, like they reflected in that election-night party, to be found in working with or alongside him.

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So far, that's mostly been in his own riding and others with large Indo-Canadian populations, including energizing appearances for NDP candidates during British Columbia's recent election. To have much chance of winning the leadership, he'll need to show he can motivate more broadly.

But Mr. Trudeau well knows the possibility, and the value, of leveraging freshness and excitement and empathy into on-the-ground engagement. Mr. Singh may prove the first rival to come along who really grasps it, too.