Justin Trudeau is casting his refusal to be pinned down on economic policy as a sign he's more prudent and fiscally responsible — and even more prime ministerial — than Stephen Harper, whom he accuses of making it up on the fly.

The Liberal leader asserted Tuesday that the calamitous plunge in oil prices demonstrates how reckless Harper's government has been in spending a budget surplus that is rapidly vanishing.

"I think it's become fairly clear that because Mr. Harper put all his eggs in the same basket, counting on oil prices remaining high, he is now reacting and flailing and indeed making it up as he goes along," Trudeau said outside the first day of a two-day Liberal caucus retreat.

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A Conference Board of Canada report Tuesday estimated that the drop in oil prices — to less than US$50 from more than US$100 a barrel last summer — will drain $4.3 billion out of federal revenues this year.

The repercussions of the oil price slide prompted Finance Minister Joe Oliver to announce last week that he's postponing the budget until at least April.

As a cornerstone of its platform for the federal election scheduled for October, the government has vowed that the 2015-16 budget will post a modest surplus after six years of deficits and it has already committed to spend most of it on a family tax-benefit plan.

However, the loss of billions in oil revenues has raised questions about the government's ability balance the budget and deliver on its pricey promises.

It's also raised questions about how NDP Leader Tom Mulcair, who began unveiling platform planks last fall, intends to pay for his proposals, including a $5-billion-a-year, national day-care program.

Trudeau has been heavily criticized for refusing to spell out policy details in advance of the election. But he cast that as virtue Tuesday, in light of the oil price plunge.

"That sense of (fiscal) responsibility also is highlighted by the fact that we didn't put out a platform last year when there were predictions of the kind of surplus we had," Trudeau said.

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"That surplus is disappearing. Mr. Harper is now facing a very different reality which has him lurching about searching for a Plan B, making it up as he goes along."

At a boisterous evening rally of party faithful later, Trudeau, a former teacher, called the government's shifting responses to the oil price collapse "the political equivalent of the dog ate my homework."

"That is weak leadership and bad judgment," he said.

Trudeau reiterated his intention to focus on expanding and bolstering the middle class with "fiscally responsible" investments in education and infrastructure, with details to come in the eventual platform.

He also repeated his promise to scrap the government's plan to allow couples with young children to save money by splitting their income for tax purposes. That would cost the federal treasury $2.4 billion a year and Trudeau and other critics maintain it would largely benefit the wealthiest 15 per cent of Canadians.

"We couldn't afford it when oil was $100 per barrel and we sure can't afford it now," Trudeau told the rally, which the party said was attended by about 600 people.

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Trudeau's message is one that Liberal strategists hope will resonate through southwestern Ontario, the province's manufacturing heartland that has been battered by factory closures. Liberals were entirely shut out of the region in the 2011 election.

"The people of southwestern Ontario are amazingly resilient and have demonstrated that moving beyond manufacturing-based employment is something they're willing to do," Trudeau said in a brief remarks kicking off the caucus retreat.

"But as is the case for so many parts of the country, they need a partner in Ottawa."

Oliver pounced on those comments to accuse Trudeau of turning his back on the region's workers, "encouraging them to close shop."

"Trudeau's high-tax, high-debt plan would devastate the industry just as it continues to recover from the global recession," Oliver added in a written statement.

But Trudeau shot back that the Harper government has neglected other regions and other sectors of the economy in its blinkered reliance on high oil prices.

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"Canada's strength shouldn't rely on any one thing or one place," he told the rally. "It's built on the diversity of our people and the diversity of our economy."

The caucus retreat is aimed at plotting strategy for next week's resumption of Parliament but also for the campaign to the next election.

Trudeau and his 34 MPs heard Tuesday that the party reaped its best fundraising haul in a decade last year, filling the Liberal war chest just in time for the election.

Party officials revealed the Liberal take for the last three months of the year topped $5.6 million, bringing the total for the year close to $16 million.

That's likely not enough to beat the Conservatives, who are on track to exceed their 2013 fundraising haul of $18.1 million. But it will take a big bite out of the Tories' fundraising advantage.