When he goes up against Finance Minister Bill Morneau in the House of Commons, Pierre Poilievre looks for what he calls "vulnerability."

That's why the Conservative Finance critic doesn't like to read from a script – so he can deliver a quip as quickly as need be.

At the height of opposition attacks over the minister's alleged conflict-of-interest, Mr. Morneau tried to turn the table on his antagonizer, asking what Mr. Poilievre holds in his own numbered company.

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Mr. Poilievre shot back: "It's a rental property. How hard was that?"

"With Morneau, the challenge is always to demonstrate the gap between what he's doing and what he's saying," Mr. Poilievre said recently, over all-day breakfast in his south Ottawa riding. "And he's made that quite easy for the last couple of months."

That a 38-year-old career politician commonly known as "Skippy" could successfully challenge the well-heeled 55-year-old former executive chair of Morneau Shepell, which describes itself as the largest administrator of retirement and benefits plans in Canada, has proved to be one of the biggest surprises of Mr. Poilievre's tenure.

First elected to Parliament at 25, Mr. Poilievre was seen as the scrappy kid, quick to mouth off, not above breaking the rules to make a fist at his political rivals across the floor or curse in committee.

Aggressively partisan, he was often trotted out to stonewall or defend Stephen Harper's government during difficult times.

In 2014, Mr. Poilievre was the minister of state in charge of changing the country's election laws. The measures – which boosted ID requirements on voting day and placed limits on Elections Canada – were decried by critics, who warned it could disenfranchise voters and benefit the Conservatives. The Tories eventually backed down on many controversial elements of the bill, but not before Mr. Poilievre received a death threat delivered to his house (not, however, his first).

"Why some people thought it was such a draconian measure, to ask people to produce some ID when they vote, I never understood it," Mr. Poilievre said.

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He was an unlikely candidate to become one of his party's most important performers in the House of Commons. But this is where he finds himself: centre stage in his role as finance critic, using that same quick tongue and relentless strategy to go after Mr. Morneau.

"When I was younger, I felt I needed to be part of every battle. Even those that were not my own," said Mr. Poilievre, who wears clear-frame glasses and combs his dark hair neatly to the side. "I made the decision to zero in on the things that matter most to me, and to leave the rest to others."

Mr. Poilievre hasn't lost his partisan edge. As employment and social development minister in 2015, he wore a Conservative Party T-shirt during two government announcements, which Canada's elections watchdog later said broke election financing rules. "I think most countries around the world would be thrilled if their leaders' scandals were related to the T-shirt they wore," Mr. Poilievre said.

But he has learned to channel his political pugilism in a more focused, measured way, to great effect against Mr. Morneau, who is being examined by the ethics commissioner to determine whether he contravened Canada's ethics laws when he introduced pension legislation that could potentially benefit his former company. "The most powerful tool in debate is to measure a person by their own standards," Mr. Poilievre said.

He wasn't expecting to get the Finance critic role.

Mr. Poilievre stayed neutral during his party's leadership race. Instead, he spent his time writing "a narrative" about the Conservative economic agenda, a topic he said he's always been passionate about. He later presented it to the Tory team, including future leader Andrew Scheer.

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The vision, which Mr. Poilievre calls "free enterprise for the underdog," rails against big government, corporate subsidies, and the notion of trickle-down economics. "That's what we need to run on in the next election," he said.

To the surprise of many, Mr. Scheer offered Mr. Poilievre the most important file in opposition. (Fellow MP Maxime Bernier, who lost the leadership by a razor-thin margin and publicly campaigned for the post, was given innovation.)

A former parliamentary secretary to the prime minister, Mr. Poilievre remains loyal to Mr. Harper. He never defended the government over obligation or pressure, Mr. Poilievre said, but because he believed it was right. "And I don't regret doing it." He also dated Jenni Byrne, one of Mr. Harper's top political operatives, for years. His current girlfriend works for Conservative MP Michael Cooper.

Mr. Scheer will put his own stamp on the party, Mr. Poilievre said. "Andrew is a likeable, relatable person. And that will be his ultimate strength."

Mr. Poilievre said Canada's political system is deliberately adversarial, so he doesn't mind his critics. "If too many people on the government side were praising me up and down I would start to think I wasn't doing my job," he said.

But his relationship with Mr. Morneau hasn't been all negative. When Mr. Poilievre was first named as critic, the Finance Minister asked to meet with him.

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"He invited me for a cup of coffee," he said. "Which I thought was nice."