The message to federal Conservatives was clear as Thomas Mulcair and Justin Trudeau donned white cowboy hats and flipped pancakes at this city's largest Stampede breakfast. Just as Prime Minister Stephen Harper can no longer count on these griddles being his alone, Western Canada may now be up for grabs.

This was the first time that both current federal leaders of the New Democrats and Liberals served the long, snaking lines of Calgarians looking for an early morning helping of two pancakes, a dollop of whipped cream and a cup of coffee at the Chinook breakfast.

The Calgary Stampede has long been a fixture on the summer schedule of vote-hungry federal leaders. However, with an unprecedented victory by Alberta's New Democrats ushering in the province's first new governing party in more than a generation, and a national economy struggling due to depressed oil prices, opposition leaders see a path to victory in October's federal election running through the Stampede grounds.

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"It may well be that this city is in play for the first time in my lifetime, so it'll be interesting to see how people respond to that," Calgary Mayor Naheed Nenshi said while perched on his horse before the start of the Stampede parade.

The political dynamics in the province are now different because of Rachel Notley's victory, Mr. Nenshi added. "That doesn't necessarily mean that Tom Mulcair will find super fertile ground here, but I think it means that Calgarians and Albertans have said that we can do different things."

With six new seats being added around Alberta's fast growing cities, the province will soon be sending its largest contingent of MPs to Ottawa. While few have suggested that Mr. Harper's Conservatives will not capture the bulk of the province's seats, Mr. Mulcair and Mr. Trudeau both see possible breakthroughs in Calgary and Edmonton.

"There's a reason I made my first campaign stop in my leadership here in Calgary," said Mr. Trudeau. "There's a tremendous appetite for change, there's an openness and fatigue to being taken for granted that we saw articulated very clearly in the provincial election. The work needs to continue to be done to demonstrate that we are that strong change."

With Canadians facing a fixed-election date in October, widespread political advertising has already started months before ballots will be cast. Despite a number of negative ads directed at him by the Conservatives, Mr. Trudeau promised again during the Stampede not to respond with attack ads of his own. "I think one of the things we've seen across the country is there is an openness to change and tiredness with the negativity and cynicism that tends to dominate politics these days," said Mr. Trudeau, who is locked in a fight with the NDP Leader to be seen as the candidate that best represents change.

Mr. Mulcair has also criticized the use of negative ads. While Mr. Mulcair was barely mentioned by the Prime Minister at his annual Stampede barbecue last year, with most barbs directed at Mr. Trudeau, both opposition leaders faced blistering attacks on Saturday evening as Mr. Harper spoke to a room of the Tory faithful.

"This October, Canadians will choose security over risk. They will choose, I have to say it, a strong, stable, national majority Conservative government," Mr. Harper said, attacking his opponents on their economic platforms and approach to national security.

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"These guys are way out there – they simply do not get it," he said. "We've come too far to take risks with reckless policies."

With his party riding high in the polls after Ms. Notley's victory in early May, Mr. Mulcair says that Canadians are now taking a second look at the federal New Democrats. Having unseated a Progressive Conservative Party that had ruled Alberta for nearly 44 years, Ms. Notley's win provided proof that the NDP is a viable alternative, he said.

A year ago, Ms. Notley was at the Stampede campaigning to be elected as the leader of the provincial NDP. Last Friday, she rode a horse in the Stampede parade, an honour not given to a premier from outside Calgary in decades. Ms. Notley said that no federal leader could take Alberta's votes for granted any more.

"What's happened here in Alberta has shown that you can chose who reflects your values. You don't need to make a strategic vote," she said. "If they agree with Tom Mulcair's NDP, that's what they should do."

Mr. Mulcair said that an NDP government in Ottawa would emulate some of Ms. Notley's initial steps in Edmonton. In her first two months in power, the Alberta Premier moved quickly to put in place a series of hikes to the province's carbon levy, announced a contentious review of energy royalties and put in place increases to the minimum wage that will see it reach $15 an hour in 2018.

Mr. Harper and Ms. Notley will hold their first formal meeting on Monday morning in Calgary. The two met quickly at a Stampede breakfast and had previously spoken on the phone. While neither has been willing to say what they intend to talk about, Mr. Harper said Saturday that Alberta's economy will rebound – even with Ms. Notley at the helm.

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"These are not the best of times for the Alberta economy," he said. "We have seen a lot worse and this province has always bounced back. Let me assure you that with Alberta's can-do attitude and a national government that is on our side, this province is going to come back."

According to Michele Austin, chief of staff to Maxime Bernier when he was industry minister, and now a senior adviser at Summa Strategies in Ottawa, the worst Ms. Notley can expect from Mr. Harper in private is a warning about moving too fast on new energy regulations and climate rules.

"I think he will wag his finger [at Notley] on the economy and say, 'You need to tread carefully there,'" she said. "Expect that they'll square off on the economy, disagree on climate change, but I don't think Harper will go too far. I don't think he'll wander into health care, minimum wage or royalties."

With a report from Sean Silcoff