And now, the hard part begins. Justin Trudeau has been Liberal Leader for less than a year, and in that time the party has gone from its deathbed to leading in the polls. Mr. Trudeau deserves much of the credit for this improbable turnaround. The power of his family name, the eagerness of voters to project hopes onto him, their willingness to overlook the thinness of his resumé, the fetish made of his perceived youth (though he is barely younger than his father was when he first came to Ottawa in 1965, already a leading constitutional scholar and activist) – all have been like helium in his balloon.

He's been helped by dissatisfaction among large parts of the electorate with the current government's policies and style, and the fact that the party he leads, a party that was temporarily so bereft of support that it looked to be prime for a going-out-of-business sale, is the most successful brand in Canadian political history, representing a middle-of-the-road identity that many voters want to be associated with. Many Canadians seem to like the idea of the Liberal Party, and the idea of Justin Trudeau. What they don't know much about are the ideas of Justin Trudeau.

The Liberal Leader has been studiously guarded when it comes to talking about how he would govern. It hasn't harmed him so far. But if he is to close the deal with voters on election day 2015, they will need to know more. He's applying for a job, and they deserve to know what he'll do if they give it to him.

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That's what Mr. Trudeau must begin to address when he speaks on Saturday afternoon, at the Liberal convention in Montreal. He won't be unveiling the latest edition of the Red Book, and he doesn't have to. Delegates and voters aren't asking for a full policy platform – yet. But he's got to start laying out the foundations, and showing us the equivalent of architectural drawings. He's got to start giving Canadians more of a sense of how he would govern, what he would seek to do with a budget of nearly $300-billion a year, and how that would or would not differ from Stephen Harper.

Mr. Trudeau's style may be very different from that of the Prime Minister, but what about his substance?

Start with the surpluses that Ottawa is about to begin running. In next spring's budget, for the first time in nearly a decade, the federal government is going to be in a position to deliver substantial new tax cuts, or significant new spending. What would Mr. Trudeau and the Liberals do? The Conservatives are on track to lower federal spending as a share of the economy to levels not seen since the early 1960s. Is that a trend Mr. Trudeau would reverse? How and why?

Trudeau has said that "growth for the middle class" will be a theme at the centre of his party's agenda. That's a fine motherhood slogan, but what does it mean? The current government believes that the middle class is best supported by a free-market economy, progressively lower taxes and targeted assistance to Canadian families. How does Mr. Trudeau's vision differ, and how would his plans be different?

Does he favour introducing income-splitting for couples, an idea that last week was tearing the Conservative Party apart, or does he have other proposals to spend the surplus? Broader-based tax cuts? No more tax cuts? Enhanced aid to working families through the tax system, or a national daycare program, funded in part by Ottawa?

And what about retirement? The Conservatives brought in a long-term plan to lower payments from Old Age Security, while declining to expand the Canada Pension Plan. Would the Liberals stick with that approach of slightly scaling back the retirement safety net, or would they seek to expand it?

What about subsidies to industry and farmers? Would a Liberal government be willing to say no to a company like Chrysler, coming cap in hand and demanding hundreds of millions of dollars in subsidies? Would Mr. Trudeau be able to stand up to the farm lobby? He's talked vaguely about the benefits of free trade – and who doesn't? – but would he be willing to deliver lower food prices to middle-class Canadians by dismantling supply management, and taking on the feared dairy farmers?

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What about education and innovation? By many measures, Canada does well on the former, and relatively poorly on the latter. What would Mr. Trudeau do to change that?

How about the environment? Mr. Trudeau has been happy to criticize the current government for dragging its heels on a carbon-reduction strategy, and for backing out of the Kyoto Protocol. But what's his alternative?

Then there's Quebec. How will Mr. Trudeau handle the looming possibility of another referendum? And how does he propose to deal with Canada's first nations? In dealing with files such as aboriginal education, the Conservatives have been tough, but they've also been pragmatic. What's Mr. Trudeau's plan?

The next federal election is more than a year and a half away. But the pre-election budget is only a year off. Come next spring, the Conservatives will be able to show the national accounts in growing surplus, allowing them to lay out a multiyear agenda of tax cuts and spending promises. Mr. Trudeau risks being steamrolled unless he can get his own policy ideas out there, long before the 2015 budget. Now would be a good time to start talking.