The stalled program to replace Canada's aging fleet of fighter jets has come to life on the campaign trail, with Liberal Leader Justin Trudeau promising to scrap the F-35 plan, and his opponents accusing him of pre-empting the procurement process and threatening the country's aerospace industry.

Sparring erupted on the weekend when Trudeau promised to exclude the F-35 from the bidding process. Instead, he said, a Liberal government would opt for a cheaper alternative that would free up more funds for navy ships.

Today, Conservative Leader Stephen Harper said the Trudeau plan would "crater" Canada's aerospace industry, while Trudeau hit back, saying Harper's "dream" aircraft would be a "nightmare" for the Canadian taxpayer.

During a campaign event in St. Jacobs, Ont., Harper called it "incomprehensible" that Trudeau would move to harm an industry that is vital to Canada's economy.

"The Liberal Party is living in a dream world if they think we can pull out of the development project of the F-35 and not lose business," Harper said. "I don't know what planet they're living on.

"Whether it's his statements on the aerospace industry, his statements on the deficit, you name it. It shows his disconnect and a profound lack of understanding about the Canadian economy."

The Conservatives announced an agreement in principle in 2010 to buy 65 Lockheed Martin F-35s, a single-engine "stealth" fighter. But the purchase plan was put on hold two years later amid growing controversies around costs and other problems. An auditor general's report accused the government of fudging cost projections without sufficient research. 

Hundreds of millions spent on F-35 development

Canada has invested hundreds of millions of dollars on the development of the F-35, money that has allowed Canadian companies to participate in contracts related to the program. That participation did not oblige Canada to actually buy any of the planes.

As of summer 2014, 33 Canadian companies had contracts worth $637 million U.S.

Trudeau: F-35 jets would be a 'nightmare' for taxpayers2:34

NDP Leader Tom Mulcair called Trudeau's move to exclude an option in the midst of a procurement process "one of the most surprising things" he has heard the Liberal leader say so far in the campaign.

He said the move showed a "total lack of experience."

"How can he decide the result in advance of the process? You can't do that. The basic rule of public administration is you define what you need, then you go to a public tender process and the lowest conforming bidder gets the contract," he said.

Mulcair said both his opponents have it wrong. While the Liberals are ruling out an option without having all the facts, Harper has practised "decision-based fact making" by pursuing the F-35 at all costs.

The NDP, in contrast, would embark on an open procurement process to get the right fighter jets in the air quickly,  he said.

"Because of years of indolence and lost advantage and potential by the Conservatives, we're in a tough bind," he said. "So we're going to try to get this right. We're going to define quickly what we need. We'll  start a process that will get us a fighter jet rapidly because our women and men in uniform need that, and Canada needs it" as part of our defence.

Trudeau said while Canada took part in the development program, it has no obligation to purchase planes that have been plagued with problems.

Problems, skyrocketing costs

"It no longer makes sense, if it ever did, to have a stealth, first-strike capacity fifth-generation fighter," he said. "There are many other fighters at much lower price points that we can use that have been proven, that we will actually be able to deliver in a timely way to replace our CF-18s and make sure our military has the planes it needs and also the ships we need to continue to be the country we expect us to be."

Dave Perry, senior security and defence analyst at the Conference of Defence Associations Institute, said the Conservative government's record has been "mixed" when it comes to military procurement, with the fighter jet file being the biggest problem in the last four years. He said it's difficult to determine the effect on the domestic aerospace industry if Canada ultimately proceeds with the F-35 or chooses another fighter.

"The aerospace industry in Canada has dozens of different tasks and hundreds of companies, not all of which has won a contract for F-35," he said. "So we wouldn't lose billions in contracts, we would forgo the potential to win them if we went a different way."

Kristen VanderHoek, spokeswoman for the Aerospace Industries Association of Canada, said as a matter of policy the association does not comment on specific government procurement programs, so she could not speak about the potential loss or gain of regional benefits attached to the F-35 decision.

The F-35 program was developed by Lockheed Martin, and was designed to promote a common system between allied partners including the U.S, Britain, the Netherlands, Italy, Turkey, Denmark, Norway and Australia.

The claims and counterclaims being cast about by the parties don't tell the entire story, however.

Trudeau did not specify how much he hoped to save by forgoing the F-35 purchase in favour of an open competition for a less-expensive plane. Background material issued by the party put the "fly-away" cost of the F-35 at $175 million per plane and the price of the rival F-18 Super Hornet at $65 million. 

But $175 million is the price for the prototype phase of the project, not the actual cost of the planes once production numbers rise, which is pegged to be about $75 million US per jet, according to Lockheed Martin.

And Harper's assertion that abandoning the F-35 would wreck Canada's aerospace industry ignores that other manufacturing firms, including F-18 maker Boeing, also offer domestic supply chain opportunities.

As the politicking over the planes continues, the company at the centre of the debate insists it has the best jet for the job.

"We are supporting the Canadian government as they work through their decision process. We believe the F-35 is the best solution for Canada's fighter defence needs," Lockheed Martin spokesman Michael Rein said in a statement to CBC News.