The two rival parties differed on precisely how deep that hole would be and on their suspicions of what Mulcair would do to dig himself out of it.
But the message was the same: the NDP leader is not being honest about the cost of his election program.
Mulcair dismissed both the Liberal and Conservative numbers as fictional and called them a sign of desperation by rivals trying to blunt the NDP's early momentum in the marathon campaign to Oct. 19. Many recent polls suggest the New Democrats are in the lead.
Liberals claimed there's a $28-billion gap between Mulcair's promises of new spending and his pledge that an NDP government could balance all its budgets over a four-year term.
Jason Kenney, the defence minister, estimated at least an $8-billion gap in the first year of an NDP government. He said that doesn't include more than 100 other promises New Democrats have made over the past three years without attaching a price tag.
"It's something that the Conservatives are making up,'' Mulcair said during a campaign stop in Halifax. "I am not going to be leaving this type of debt on the backs of future generations. I'll leave that to Mr. Harper and Mr. Trudeau.''
Mulcair said the Conservatives' cost estimate includes "things that have been presented long in the past'' by the NDP, and which apparently no longer apply.
"It's an attempt to distract from things that are real, like the fact that Stephen Harper has run up $150 billion in debt while he's been the prime minister of Canada,'' said Mulcair.
As for the Liberals' estimate of a $28 billion gap, he added: "That one by the Liberals is so fanciful that it defies description.''
But Liberal MP John McCallum, a former bank economist, called on Mulcair to produce his own math.
"He won't come clean about his math because the math doesn't add up. We know because we did the math for him,'' McCallum told a news conference.
"Tom Mulcair is not telling the truth to Canadians. He's offering a phoney set of promises that he has no intention of keeping.''
McCallum said Mulcair would have to slash spending or break most of his promises if he's serious about balancing the budget next year.
But Kenney posited a third possibility: that Mulcair is secretly planning to impose massive tax hikes. And Kenney said he suspects the NDP is planning a carbon tax, what he called "a tax on everything.''
"Canadians cannot afford the NDP,'' Kenney told a separate news conference.
"We're only a third of the way through this campaign and already their reckless spending would mean massive tax hikes.''
Experts are, in fact, divided over whether the Harper government will be able to balance this year's books as promised. The parliamentary budget officer has predicted a $1.1 billion deficit, due largely to the steep dive in oil prices.
Liberal Leader Justin Trudeau last week set himself apart from the rest of the political pack, announcing that he would run deficits of up to $10 billion for two years in order to stimulate economic growth and job creation.
Among other things, Trudeau is promising to invest $60 billion in infrastructure.
Mulcair has promised to eventually release a full costing of the NDP's platform.
Neither the Liberals nor the Conservatives have yet released their own platforms with full costing. But that hasn't stopped them from doing their own math on the NDP's yet-to-be-released numbers.
Kenney said his party used "the most conservative estimates possible,'' based on numbers provided so far by the NDP, as well as the parliamentary budget officer.
The Liberals said they too used NDP numbers, fiscal projections for the next four years from the last federal budget and the parliamentary budget officer's most recent fiscal outlook.
Both parties have made some assumptions which may or may not be accurate. For instance, the Liberals assume Mulcair would generate $460 million in revenue in his first year as PM by raising corporate taxes; the Conservatives estimate an NDP corporate tax hike would raise $3.7 billion in the first year.
Mulcair has not yet specified how high he'd raise corporate taxes, other than to say the rate would remain "far below'' the average under the Conservatives of 17.5 per cent.
— With files from Aly Thomson in Halifax
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