Justin Trudeau says a Liberal government won't balance the books for three straight years but will double spending on infrastructure to jump-start economic growth.

The Liberal fiscal plan would see "a modest short-term deficit" of less than $10 billion for each of the first three years and then a balanced budget by the 2019-2020 fiscal year.

The Liberals also announced they would commit billions to nearly double Canada's spending on infrastructure, part of the party's pitch to create jobs and stimulate economic growth.

The decision not to balance the budget immediately opened Trudeau to a barrage of fresh criticism from Conservative Leader Stephen Harper, who accused the Liberal leader of "changing his tune."

Justin Trudeau climbs into a crane in Oakville, Ont. ahead of an announcement Thursday. The Liberal leader pledged to double infrastructure spending if elected - and said he'd run short-term deficits to do it. (Catherine Cullen / CBC)

Trudeau, making a campaign stop in Oakville, Ont. Thursday morning, said over the next decade the Liberals would spend $125 billion on new infrastructure investment — about twice the amount the Conservatives have committed.

Investments would focus on three areas: public transit, social infrastructure such as affordable housing and seniors centres and "green" projects like clean energy infrastructure.

"Our economy needs investment in order to create growth," he said. "Our plan features three years of historic investment in the Canadian economy. That growth will eliminate the Harper deficit and we will balance the budget in 2019," he said.

Asked if he would deliver on campaign promises even if the economic situation deteriorates, Trudeau said he would be "open and transparent" with Canadians about the state of the economy.

Speaking in Hamilton later in the day, an animated Stephen Harper mocked the Liberal leader's announcement after Trudeau previously said "the budget will balance itself" — a refrain used by Conservative attack ads that have swarmed the airwaves for weeks.

Conservative leader Stephen Harper reacts to Liberal leader Justin Trudeau's plan to run deficits. 1:18

"He now says he'll run a modest deficit, a tiny deficit, so small you can hardly see it," Harper said, pinching his fingers slightly and drawing laughter from the crowd of supporters. "And only for three years, so three modest, little deficits, and maybe $10 billion each, a modest, $10-billion deficit to start with.

"Friends, we've gone through this before. Look at the mess in Ontario with a modest deficit from a Liberal government."

Liberals, NDP trade jabs over deficits

Trudeau, who has harshly criticized NDP Leader Tom Mulcair for saying an NDP government would balance its first budget, said Mulcair is being "disingenuous" by promising to balance the budget and still pay for commitments like more affordable child care spaces. Trudeau insists cuts will be required under the NDP plan.

Liberal supporters crash an NDP event in the Toronto area riding of Eglinton-Lawrence. NDP candidate and former Saskatchewan finance minister Andrew Thomson has said cuts would be "inevitable" if an NDP government were elected. (Tom Parry/CBC)

Mulcair argues he is offering a more balanced approach than his adversaries. He hasn't denied cuts will be required but has only provided a few examples of how it will be done — such as eliminating the Conservative measure to offer income splitting for families.

At a campaign stop in Toronto Thursday, Mulcair said the days of downloading big debts on the provinces would be over under an NDP government and offered a few more examples of the party's spending priorities.

"There's a whole bunch of things we're going to do differently from Stephen Harper. He subsidized oil companies to the tune of billions of dollars while he's been in power. We won't do that. He spent millions fighting First Nations. We won't do that. He spent a billion dollars on the Senate. I`m going to try and make sure Canadians never spend another penny (on the Senate)."

Tom Mulcair says and NDP government would balance the budget in part by eliminating Conservative programs. 1:13

Trudeau has also argued that Harper's own record of deficit spending means the Conservatives can't be trusted to balance the budget if re-elected.

But Harper touted his own economic credentials when it was pointed out that he himself used deficits to combat the recession in 2009.

"My economic training tells me that's actually the time when you do run a deficit," Harper said. "When you have modest growth,we have modest growth this year, we will have modest growth, you don't return the country into deficit."

In an email to CBC News, Conservative Party spokesman Stephen Lecce said the Harper government has increased infrastructure investments by three times what the former Liberal government invested — all while cutting taxes and balancing the budget.