Liberal Leader Justin Trudeau, in the first question of the night from the moderator, Maclean's political editor Paul Wells, was quick to mention the word recession, deeming it to be a risk to continue with Harper's "failed plan."
Both Trudeau and NDP Leader Tom Mulcair brought up the spectre of consecutive deficits on more than one occasion.
"Honestly, Mr. Harper, we really can't afford another four years of you," Mulcair said.
Harper, referring to the Liberal leader as Mr. Trudeau after an early campaign approach of calling him "Justin" at nearly every turn, said Canada should stay the course during challenging times with his party's policy of low tax rates.
Harper trumpeted Canada as having the strongest economic growth and job creation record in the world since the great global recession began in 2008. He also defended his record on ensuring Canadians have income stability in their senior years, pointing to income splitting for pensioners.
Mulcair and Trudeau are participating in their first federal election debate.
Green Party Leader Elizabeth May accused Harper of "cherry picking his data" and called him on statements made in a 2008 federal debate in which she participated.
"I don't think he's got a really good track record of spotting when we're in a recession," she said.
Trudeau criticized Harper for his lack of co-operation and partnerships with First Nations on environmental issues and with the provincial premiers on a host of issues.
With respect to the proposed Keystone XL pipeline project that would ship Alberta oilsands crude to the U.S., Harper pointed to overwhelming support from the public and among politicians south of the border, other than the current U.S. president.
He contended that the Conservatives are the first government in Canada to reduce greenhouse gas emissions, which drew an immediate rebuke from May as being untrue.
May said the government had "no credibility at this point" with respect to the environment, and that it was not sound policy to export crude to countries with poor environmental records.
Mulcair, too, took Harper to task for projects that would be more beneficial to Canada's partners than domestically.
"Part of sustainable development is creating value-added jobs in your own country," he said.
Trudeau accused Mulcair of inconsistency on the issue, and in addition said that too often Harper's policies treat the economy and environment as separate issues.
Harper shot back that by opposing carbon taxes, unlike the opposition parties, he is supporting Canadians at the pumps and in their homes.
Tonight's debate will also include segments on the health of Canada's democratic institutions (such as the Senate); and foreign policy and security.
Trudeau is the fourth Liberal Party leader Harper has faced in a federal debate since taking office, and Mulcair is the second NDP leader he has faced. May participated in the 2008 consortium debate, but was excluded three years later.
Future debate questions
More than two months away from voting day, the NDP has a narrow lead in the polls, with the Conservatives second and the Liberals third.
While it is only five days into the longest federal campaign in modern Canadian times, this could be the only English-language debate to feature all four major party leaders.
There are expected to be four or five more debates, but the lineup card for each is still the subject of some posturing and negotiation.
Harper, Prime Minister since early 2006, has said he will not participate in the traditional debates organized by the broadcast consortium, of which CBC is a member. Harper has agreed to the Maclean's and TVA debates, as well as those proposed by the Globe and Mail and the Munk School of Global Affairs.
Mulcair has suggested he won't attend debates in which Harper does not participate.
Voters head to the polls on Oct. 19.