The NDP has to account for a multibillion-dollar hole in its platform spending promises, the Conservatives and Liberals said Sunday.

The rival parties laid out what they say are significant discrepancies between NDP Leader Tom Mulcair's commitments of new spending and his pledge that an NDP government would not run a deficit — though they differ on how large the hole would be and what Mulcair would have to do to dig himself out of it.

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.

Liberals claim there's a $28-billion gap, over a four-year term, between Mulcair's promises and his claim that an NDP government would be able to balance the budget.

"He won't come clean about his math because the math doesn't add up. We know because we did the math for him," Liberal MP John McCallum said.

Meanwhile, Conservative candidate Jason Kenney said there's at least an $8-billion gap for just 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.

McCallum and Kenney made their remarks during separate news conferences at the National Press Theatre in Ottawa.

McCallum, a former bank economist, said Mulcair would have to slash spending or break most of his promises if he's serious about balancing the budget, adding that "Tom Mulcair is not telling the truth to Canadians. He's offering a phoney set of promises that he has no intention of keeping,"

But Kenney accused Mulcair of secretly planning to impose massive tax hikes.

"Canadians cannot afford the NDP," Kenney said. "We're only a third of the way through this campaign and already their reckless spending would mean massive tax hikes."

'A sign of desperation'

Mulcair has promised to eventually release a full costing of the NDP's platform, detailing where he'd get the money to pay for all his commitments.

Speaking to reporters after a speech in Halifax on Sunday afternoon, Mulcair said both the Conservative and Liberal calculations included remarks he made on spending before the current campaign.

"What they came up with is not a reference of what we've been talking about in this campaign. They're looking at, you know, things that have been presented long in the past and has nothing to do with what we're presenting in this campaign," he said, adding that the rival parties' Sunday news conferences were a "sign of desperation."

He repeated that the NDP has been very clear about how it plans to pay for its platform promises.

NDP Leader Tom Mulcair was in Halifax Sunday for a rally, where he reiterated his promise to roll back the retirement eligibility age from 67 to 65. (Darren Pittman/Canadian Press)

"Whether it's $15 a day child care or any other undertaking we've put forward, we've always explained exactly how much it was going to cost, we said every step of the way what changes we would make on the revenue side."

Mulcair said in addition to new revenue, the NDP's promised tax cuts for small- and medium-sized businesses would help to kickstart a stalling Canadian economy.

"We're going to bring in our own priorities, but every step of the way, it's going to be costed, it's going to be realistic," he said. "I thought that the Conservative number was ridiculous. That one by the Liberals is so, so fanciful that it defies description," he added.

Corporate tax hike

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.

Read the document the Conservatives distributed to media Sunday: