The Liberals and the NDP took turns attacking the government's fighter jet procurement plan Wednesday in a campaign that has increasingly zeroed-in on Defence Minister Peter MacKay.

Liberal House Leader Marc Garneau questioned whether MacKay could read or whether he was just "not too bright" while NDP Defence Critic Dave Christopherson said the government's credibility was disintegrating.

"(MacKay's) got a serious credibility problem, the prime minister has a credibility problem. In fact the entire government has a credibility problem and it's time for the government to step up to the plate and provide the answers Canadians deserve," Christopherson said at an Ottawa news conference.

Last week's report from Auditor General Michael Ferguson took issue with the government's lack of transparency on the purchase of 65 F-35 stealth fighter jets sought to replace the aging fleet of CF-18s.

Ferguson also said cost estimates were off by as much as $10 billion and the decision to purchase the jets was made long before the plan was presented to the Canadian public.

MacKay said Tuesday in Halifax that the auditor general and the government simply used different formulas to arrive at their cost estimates.

While Ferguson's estimates included operational costs, such as fuel and pilot's pay over the lifespan of the jets, the Defence Department's did not.

MacKay said on Tuesday the cost estimates for 65 F-35 fighters would be significantly higher if the "new way of doing business" for future procurements required full operational cost estimates.

Garneau and Liberal defence critic John McKay said Wednesday they didn't buy the explanation and said MacKay didn't seem to be up to speed on his own file.

The comments came after a gaffe on Tuesday when MacKay referenced the wrong table in the auditor general's report while trying to make his case over the conflicting numbers.

Garneau said MacKay is either incompetent or "not too bright."

He accused the Conservatives of lying about "skyrocketing costs" for the planes, about having a contract that would insulate Canada from those rising costs, and about holding an open competition to choose the company to build the jets Canada needs.

"They have written a blank cheque with Canadian tax dollars and are now trying to duck accountability by calling their $10-billion blunder an accounting error," Garneau said.

Interim Liberal leader Bob Rae issued a written statement on Wednesday attacking the Conservatives.

"This is a failure to come clean with the Canadian public when the Prime Minister had countless chances to do so. The responsibility for this incompetence lies with Stephen Harper and he must resign," Rae said in a statement.

Garneau and McKay also demanded that Harper resign, along with MacKay, and said the government should "stop blaming soldiers and bureaucrats" and hit the "reset button" on the procurement process.

"Defence Minister Peter MacKay blames the $10 billion hole on 'accounting' mistakes," said McKay. "This giant gap is not a mistake, it's a failure of leadership that extends all the way to the Prime Minister's Office."

The NDP didn't go quite so far. Christopherson said he wants more information before calling for MacKay's head, but said he questions why the government took days to explain away the cost discrepancy as an accounting issue.

"I don't buy that," he said. "I don't buy that for a moment because at the time that would have been a reasonable answer to the discrepancy but they didn't do that, they went on the attack."

Canada announced in July 2010 it would purchase the planes as part the U.S.-led Joint Strike Fighter Program. The U.S. had decided on the Lockheed Martin F-35 after a competitive bidding process between Boeing and Lockheed Martin.

However, Ferguson said in his report the F-35 was the plane the government and Defence Department wanted as far back as 2006.