OTTAWA -- With the possibility that Canada could join a massive Pacific Rim trade deal, Tom Mulcair tried to cast his NDP on Sunday as the only party prepared to stand up to the Conservatives on a pact that could alter the landscape of the 78-day campaign.

While Conservative Leader Stephen Harper kept a low profile, Mulcair hit the hustings hard, blazing through southwestern Ontario in an effort to take six ridings held by the Tories.

The NDP leader made it a point to campaign in a region that could be acutely affected by the Trans-Pacific Partnership, as he did Saturday when he stopped in the heart of Quebec's dairy country. Ontario's auto industry has been battered in recent years as companies increasingly set up shop in Mexico, where labour costs are cheaper.

At rallies throughout his whistle-stop tour, Mulcair said that a vote for the Liberals would be no different than a ballot cast for the Tories, particularly on the issue of the TPP -- a deal he insisted could hurt Canadian farmers and manufacturers.

"The NDP, when we form government on Oct. 19, will not be bound by this secret agreement that Mr. Harper has been negotiating," he said while in Brantford, Ont., to cheers.

"There's nothing new in the Liberals backing the Conservatives. We saw them do that on Bill C-51, which was the biggest attack on our rights and freedoms since the War Measures Act. Who stood up to the War Measures Act? Tommy Douglas and the NDP. That's standing on principle. Who's the only party that had the courage to vote against Bill C-51? The NDP."

There was much suspense Sunday in Atlanta, where negotiators worked around the clock for a fifth consecutive day in a bid to reach an agreement that would include 12 countries and create the world's largest trade zone.

Mulcair has been trying to burnish his social-democratic credentials by promising he won't be bound by a Conservative deal. He has hammered away on the importance of upholding the country's supply management system for dairy and poultry farmers.

Foreign negotiators have said Canada's protectionist system would have to be opened up for a deal to be reached, though it remains unclear what concessions the federal government is willing to make.

The Conservatives have insisted they would keep the supply-management system intact amid political pressure from opponents, provincial governments and the dairy lobby. But the federal government has left open the possibility it could allow for an additional, if limited, share of foreign goods into Canadian grocery stores.

A TPP deal, which the government has said would boost the Canadian economy as a whole, could also have a similar effect on the auto sector.

Liberal Leader Justin Trudeau, meanwhile, said it would be premature to weigh in on the issue without knowing the details of a possible deal.

"I'm not going to speculate or hypothesize on this," he said while emphasizing that his party is pro-trade.

Earlier in the day, Trudeau held what the Liberals billed as their largest rally of the campaign at a sports and entertainment complex in Brampton, Ont., on the outskirts of Toronto.

He told the several thousand packed inside the Powerade Centre that Tory voters have similar interests as their own.

"Conservatives are not our enemies. They're our neighbours," he said before a crowd that included former prime minister Paul Martin.

"We don't need to convince them to leave the Conservative party. We just need to show how Stephen Harper's party has left them."

Harper had no scheduled public events Sunday. But he was to campaign Monday in the nearby town of Richmond Hill, Ont., where he would certainly try to sell the merits of any TPP deal.