OTTAWA -- On the eve of the first federal leaders' debate, Canada's campaign trail grew quiet Wednesday as the rivals prepared for their live four-way faceoff.

Two of them, however, sent campaign proxies.

Laureen Harper stepped up to fill her husband's shoes, speaking briefly to party supporters at the campaign office for Conservative Costas Menegakis in the suburban riding of Richmond Hill.

And the NDP offered a new attack ad to fill the campaign void -- a collection of doom-and-gloom business-page headlines superimposed over photos of Stephen Harper, denouncing the Tory economic record.

Indeed, the economy is expected to be front and centre during Thursday's debate, the first opportunity for four of the five leaders to go head-to-head on a variety of key issues.

All four participants -- Harper, NDP Leader Tom Mulcair, Liberal Leader Justin Trudeau and Green party Leader Elizabeth May -- were keeping a low profile Wednesday as they took part in the time-honoured campaign ritual of "debate prep."

Mulcair has been running through debate preparation sessions with his inner circle at a Montreal hotel all week, surfacing only Tuesday to take part in a campaign event in the riding of Mount Royal.

As a former prosecutor whose question-period inquisition of Harper at the height of the Senate scandal was a highlight of his tenure as leader, Mulcair's performance will be under the microscope, observers say.

"I think the question is whether in the debate, he can come across in the ... prime ministerial sense, given that now that's a real possibility for him," said Toronto-based debate coach Shakir Rahim, who won the World Individual Debating and Public Speaking Championships two years in a row.

As is always the case for the prime minister, Harper will surely be the target of most of the attacks -- a fact not lost on the Conservative team, said spokesman Kory Teneycke.

But the Tories will have attacks of their own -- particularly on matters of experience and leadership.

"Attacking the prime minister is one thing," Teneycke said Tuesday in Toronto. "What would you do if you were prime minister is another, and so I think there will be some accountability there as well. "

Teneycke said the prime minister is "quite prepared to defend" the government's economic record, which has been under heavy fire throughout the first few days of the still-young campaign.

Trudeau will exceed expectations "if he comes on stage with his pants on," Teneycke said -- a jab to which the Liberals did not respond Wednesday.

Campaign debates always feature a heavy dose of boxing metaphors, so it's hardly surprising that Trudeau, who famously defeated former Conservative senator Patrick Brazeau in a boxing match in 2012 -- will visit a boxing gym in Toronto ahead of the event, television cameras in tow.

Mulcair, meanwhile, will pay a visit the office of NDP candidate Jennifer Hollett in the Toronto riding of University-Rosedale.

"Mr. Mulcair hopes to convey his message of change to Canadians while facing strong opponents," said NDP principal secretary Karl Belanger, "including a very experienced prime minister and a seasoned drama teacher."

Green party Leader Elizabeth May spent Wednesday on her own in Toronto ahead of the debate.

"I'm doing the same thing to prepare that I did in 2008, which was the only other time I was ever included in the national televised leaders' debate," May said in a phone interview.

"Of course, this isn't that debate."

May said she remains hopeful that the NDP will agree "to what we had previously agreed to, which was all the opposition parties and CBC, Radio-Canada, CTV and Global broadcasters" would go ahead with the debate in October without Harper, who has refused to participate.

The NDP said it agreed in principle to many of the debate proposals it has received, including those submitted by the consortium of broadcasters, but will make a final decision based on criteria including that "the prime minister and other political party leaders are invited and have agreed to participate."