A year ago, Justin Trudeau was the chosen one. Travelling through shopping malls and university campuses across the country, he drew crowds of all ages with his message of political renewal and a constant smile. The Liberal leadership contest was his to lose – and he won big.

His prize for winning the race was a moribund party that had been on a steady decline since 2004, marked by infighting and a decaying brand. He has quickly overseen a turnaround, grabbing strong poll numbers both for the party and himself, adding millions to party coffers and attracting a strong stable of would-be candidates across the country.

Now the hard work begins. In a wide-ranging interview with The Globe and Mail this week, Mr. Trudeau laid out the pillars he sees as economic priorities if he moves into 24 Sussex – education, trade and infrastructure – and made it clear that he sees a return to a bigger, more interventionist federal government. Inside his parliamentary office, Mr. Trudeau was relaxed and forthcoming – touching on everything from his marriage and growing family to the legacy of his famous father.

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For Mr. Trudeau, the political year starts on Thursday, as the Liberal Party holds a four-day policy convention in Montreal that has drawn at least 3,000 delegates. If 2013 was the year he introduced himself to Canadians, then 2014 is the year he must address the skeptics.

"The challenge and the responsibility for this year and over the next year and a half is to pick the team … and build the plan," Mr. Trudeau said. "And always get the big things right."

Liberals hope to start transforming Mr. Trudeau's relatively vague pronouncements – helping the middle class and spurring economic growth – into a concrete electoral program. While he seemed a natural heir to the Liberal crown, Mr. Trudeau now has to convince a majority of the electorate that he has better solutions to their economic woes than the current Prime Minister and the Leader of the Official Opposition.

While saying his plans for Liberal renewal prevent him from imposing his views on his followers, he made it clear he doesn't share what he calls the Conservative Party's obsession with a "trickle-down" economic model based on tax cuts. Instead, he wants the federal government to create the conditions for long-term growth in the country.

"Canadians are tapped out, provinces are tapped out," Mr. Trudeau said. "The federal government, because of smart decisions taken in the 1990s, has a little more leeway. Our debt-to-GDP ratio is great. So what we need to be looking at is how we can leverage the fact that we do have a little more wiggle room on the federal side to actually get our economy growing the right way."

This week's Conservative budget marked the start of a heated battle, with all three party leaders setting their sights on the 2015 general election. And Mr. Trudeau has two ferocious adversaries in front of him: Prime Minister Stephen Harper and NDP Leader Thomas Mulcair. As Mr. Trudeau works to define his policies, the Conservatives are working to define him, too – and feel they have struck a chord by attacking him as being "in over his head."

Mr. Trudeau will give two speeches to the Liberal convention. On Thursday, he will welcome the delegates to his hometown, and then take aim at the Conservatives in what is billed as a speech with a "partisan edge." In his keynote address on Saturday, Mr. Trudeau will expand on his plans for the country, with a meatier approach to policy. The speech will be his biggest since he won the Liberal leadership, and will serve to test his team's assertion that he has improved on every level since then.

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The convention will be launched with a conversation between Liberal MP and former journalist Chrystia Freeland and former U.S. Treasury secretary Larry Summers, setting the stage for the gathering with an open discussion on "what it takes to create economic growth that benefits everyone."

Since becoming leader, Mr. Trudeau has been consulting with businessmen, bankers, academics and former politicians on economic matters. The list is not entirely public, but he has had discussions with former New Brunswich premier Frank McKenna, senior Bay Street figures and academics Mike Moffatt, Kevin Milligan and Chris Ragan.

The convention will also feature would-be Liberal candidates, such as retired lieutenant-general Andrew Leslie who wants to run in Ottawa-Orléans, Liberal premiers Kathleen Wynne (Ontario), Stephen McNeil (Nova Scotia) and Robert Ghiz (Prince Edward Island), and Mélanie Joly, a 34-year-old rookie politician who finished in second place in last year's Montreal mayoral race.

One of the goals will be to drive home the message that Mr. Trudeau is bringing generational change to the party. The point will be clearly made in coming weeks when Mr. Trudeau and his wife, Sophie Grégoire, welcome their third child. Winning the leadership has taken an obvious toll on Mr. Trudeau's home life, and to help adjust, everyone moved from Montreal to Ottawa last year.

Sometimes Mr. Trudeau takes on a different tone, sounding less like a politician reciting prepared lines, and more like a working parent struggling to find time for his kids. He does not want politics to take the toll it exacted on his father, former prime minister Pierre Trudeau.

"My dad's focus on his country and his kids was one of the factors that led to the breakup of his marriage," he said. "I'm very mindful that my partnership with Sophie is a key part of my success as an individual, as a person. Having someone like her to keep me grounded, focused, doing yoga and not just boxing – literally and metaphorically – is absolutely essential to me."

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The "boxing" comment refers to one of his sporting hobbies, but also his constant battles with the Conservatives. On some issues, however, he is treading the same territory as his adversaries, hoping to outperform them on familiar grounds such as trade and the development of Canada's natural resources. The strategy risks irking some left-of-centre Canadians, but also stands to attract so-called red Tories to his team.

In the interview, Mr. Trudeau chastised the Conservatives for failing to persuade the Americans to approve the Keystone XL pipeline or to reassure British Columbians about the prospect of shipping bitumen through the province on its way to Asia. While Mr. Trudeau opposes the Northern Gateway project in B.C., he is "open" to the proposed Kinder Morgan pipeline, as long as Ottawa improves its approval process.

"These are key drivers of economic growth, but right now the Conservative government's approach has not been to reassure people that trade is good for us and to reach out and build those relationships," Mr. Trudeau said. "Mr. Harper has demonstrated that he is not very good at working with anyone who doesn't share his ideology. That is limiting the kind of growth that Canadians can have."

Mr. Trudeau offered encouraging words about a resolution that will be debated at the convention, which would commit the federal government to overseeing a pan-Canadian increase in infrastructure. The idea stands to prove a hit with municipalities, which are calling for more federal money to overhaul their crumbling roads, but also to build strategic infrastructure.

However, Mr. Trudeau made it clear that his government's first priority would be to increase spending on education, both in terms of transfers to the provinces and through a renewed job-training agenda. He is stealing a page from the Conservatives on this front, but he said his government would achieve greater results with a willingness to hold fruitful negotiations with the provinces, instead of the current confrontational approach.

Mr. Trudeau has capitalized on some signature moments as Liberal Leader, and stumbled on others. He has shown an ability to shake up politics by triggering a continuing debate on the legalization of marijuana, taking the lead in slamming the Quebec government's proposed Charter of Values, and expelling all senators from his caucus. However, he remains unsure at times in the House of Commons. He once went off-message as he praised the Chinese system of government, and raised questions about his judgment by taking paid speaking engagements with non-profit groups after he became an MP in 2008.

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Still, Mr. Trudeau said the coming convention is only one step in the development of the Liberal platform. He rejected the accusation that he is shying away from wading into major issues, as the Liberals argue it would be foolish for their party to do anything but slowly unveil their platform planks between now and the next election.

"I'm not going to pretend to know what kind of fiscal room we are going to have a year and a half from now in the 2015 election," Mr. Trudeau said. "But I think that Canadians do want to know what kinds of priorities, what kinds of focus that I have."