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.
NDP AD: "Not working" – Stephen Harper promised to strengthen the economy. Let’s look at the record. #Ready4ChangeAugust 5, 2015
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 a lot of the Canadian electorate has seen Mulcair in question period on the attack, which is obviously a useful quality to have,'' said Toronto-based debate coach Shakir Rahim, who won the World Individual Debating and Public Speaking Championships two years in a row.
"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.''
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.
"We are focused on getting ready for the debate,'' Teneycke said Tuesday in Toronto.
"Attacking the prime minister is one thing; 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'' -- a jab at one of Trudeau's occupations pre-politics.
Green party Leader Elizabeth May is spending Wednesday on her own in Toronto ahead of the debate. May's officials say she prefers to be left alone to study, rather than participate in mock debates like the other leaders.
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