OTTAWA — Conservatives and Liberals ganged up Sunday on perceived frontrunner Tom Mulcair, claiming there’s a multibillion-dollar hole in the NDP leader’s election platform.

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 is not being honest about the cost of its 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.

Neither the Liberals nor the Conservatives have yet released their own platforms with full costing. But that didn’t stop them from doing their own math on the NDP’s yet-to-be-released numbers.

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 are apparently no longer part of the program.

“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, he said their estimate “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, calling the NDP platform cost “the biggest, darkest, deepest black hole imaginable.”

“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 suggested a third possibility: that Mulcair is secretly planning to impose massive tax hikes — in particular a carbon tax, which Kenney 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.”

He contrasted that with Stephen Harper’s record of balancing this year’s budget while lowering taxes.

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 announced that he would run deficits of up to $10 billion for two years in order to stimulate economic growth and job creation.

In calculating the NDP platform cost, 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.

But the NDP immediately pointed to several errors in their rivals’ calculations.

For instance, both the Liberals and Conservatives count $1.3 billion in the first year for Mulcair’s public transit plan. While he has indeed promised $1.3 billion in annual funding for transit over 20 years, the NDP’s fine print shows funding would only increase by $420 million in the first year.

The Liberals count some $15 billion over four years to fund Mulcair’s promise to boost foreign aid to 0.7 per cent of GDP. But the NDP campaign points out that Mulcair has not yet spelled out how quickly he intends to meet that target.

Both parties have also made some assumptions which may or may not be accurate on things like how much revenue Mulcair could generate by hiking the corporate tax rate. Mulcair has not yet specified how high he’d raise corporate taxes.

— With files from Aly Thomson in Halifax