If not Harper, who? The two left-of-centre candidates are fighting for undecided voters
Canada election 2015
If there were any doubts that this has become a tight, hard-fought Canadian general election campaign, that went out the window very early during Thursday night's leaders' debate in Calgary.
It was a spirited, sometimes snippy affair that often seemed to spin out of control, as the back-and-forth between the three candidates - Conservative Prime Minister Stephen Harper and left-of-centre challengers Justin Trudeau of the Liberal Party and Thomas Mulcair of the New Democratic Party - descended into cacophony.
The debate's moderator, Toronto Globe and Mail editor-in-chief David Walmsley, pointed out in his introduction that this was only the second time a leaders debate had been held west of Toronto.
Its location - in the heart of Canada's oil and gas country - gave a particular sense of urgency to what was tabbed as the focus of the discussion, the economy.
The entire province of Alberta has been hit hard by job losses resulting from the sharp decline in oil prices, and its struggles have become a drag on the entire Canadian economy, which entered into recession earlier this year.
For his part, Mr Harper was single-minded in his focus on the danger he says his opponents pose to what he termed a "fragile economy".
Stephen Harper speaks to a steel manufacturer in Ontario
He said Mr Trudeau's proposal for three years of deficit spending to fund increased infrastructure investment threatens out-of-control budget gaps. He accused Mr Mulcair, on the other hand, of risking harm to the economy with his plan to raise corporate taxes.
Mr Mulcair responded with a sharp criticism of Mr Harper's handling of the economy, which he said had become overly reliant on an energy sector that was bound to eventually falter.
"Mr Harper put all of his eggs in one basket," he said, "and then he dropped the basket."
Thomas Mulcair shakes hands with aerospace workers as he makes a campaign stop in Montreal
As for Mr Trudeau, he employed the refrain, "Are you better off now than you were 10 years ago?" A reference to when Mr Harper's record-setting stretch as prime minister first began.
He added that Mr Harper had overseen the worst Canadian economic growth rate since the Great Depression.
Mr Harper had a reply at the ready, however, citing his stewardship of the Canadian economy during a time of global turmoil.
"In the last 10 years, where would you rather have been than Canada?" he asked. "Looking forward, where would you rather be but Canada?"
Justin Trudeau arrives at a campaign event in Ontario
It was a theme Mr Harper would return to again toward the end of the debate, when he noted: "I have come to work seven years in a row with nothing but economic crises all of the world."
Although the debate's focus was the economy, jobs and taxes weren't the only terrain the three candidates fought over. Environmental policy also came up, and the immigration issue led to one of the more heated exchanges.
"These guys would have had, in the last two weeks, us throwing open our borders and literally hundreds of thousands of people coming without any kind of security check or documentation," Mr Harper said when asked about the recent Syrian refugee drama and how much Canada should do to take in the displaced.
He called such suggestions "an enormous mistake".
Both Mr Trudeau and Mr Mulcair responded by accusing the prime minister of fearmongering.
"Stop using the security excuse as a pretext to do nothing," Mr Mulcair snapped.
Mr Harper plays to the "fears of others, fears of different communities," Mr Trudeau said. "That's not right, sir."
According to Stephen Carter, a Calgary based political strategist, the real action in Thursday night's debate, however, was the shots traded between the two left-of-centre candidates.
In one particularly memorable exchange, Mr Trudeau described Mr Mulcair's universal childcare programme as amounting to ineffective "puffs of smoke".
Liberal and Conservative supports outside the debate venue on Thursday
"You know a little about that, don't you, Justin," Mr Mulcair quipped in a not-so-subtle reference to the Liberal leader's controversial support for marijuana decriminalisation.
The reason why the Mulcair-Trudeau confrontations were so critical, Carter says, is because although it's a three-way race, the battle over the nearly third of Canadian voters who are undecided is really only a fight between the two left parties.
Those on-the-fence voters have long since given up on the incumbent Conservatives.
Those opposed to Mr Harper still must decide between his rivals
"I thought we saw a debate between Mulcair and Trudeau only," he said.
"Stephen Harper sat on the side, letting them tear each other apart, without recognising that they weren't just tearing each other apart, they were defining the debate for the final 30 days of the election."
Mr Trudeau's objective was to dispel the notion that he's a political lightweight - a theme that the Conservatives have been hammering home in a recent advertising campaign. The expectations for him were low, and he likely more than met them with his debate performance.
The goal for Mr Mulcair was somewhat trickier. The NDP has never held power in Ottawa, and he had to assure voters that he would be a steady hand at the tiller - a task he appeared to accomplish as well.
Now Mr Mulcair and Mr Trudeau will have to seize on whatever momentum they can coming out of this debate and advance themselves as the best alternative to Mr Harper.
"After 150 years of being told we have no choice," Mr Mulcair said, "for the first time in our history there is another choice".
Mr Harper put forward a steady effort on Thursday night that contrasted with Mr Muldair's sardonic barbs and Mr Trudeau's occasionally breathless speaking style.
It's becoming clear, however, that he's going to have to thread an exceedingly narrow electoral needle to keep the government in his party's hands. If those undecideds end up breaking en masse toward either the Liberals or the NDP, he'll likely be packing his bags after the 19 October election.