With the shadow of a massive and controversial trade deal looming over the campaign trail, Tom Mulcair visited a farm Saturday to press his contention that only a New Democrat government would stick up for Quebec's dairy farmers.

At a dairy farm in Upton, Que., east of Montreal, Mulcair denounced Conservative Leader Stephen Harper for negotiating the Trans-Pacific Partnership in secret and promised to stand up for farmers and auto workers — the two sectors most worried about the would-be deal.

"Quebecers have a clear choice in this election: An NDP government that will take a strong stand in support of family farms and the dairy industry or a Conservative government that won't," Mulcair said, iconic Holstein cattle on display in the background.

An NDP government would refuse to ratify the TPP, which hasn't been negotiated in good faith, the NDP leader repeated.

"I will not put before the House of Commons an agreement that sacrifices the family farms of hard-working people across Canada who right now are prospering under supply management."

Hours earlier in Montreal, Harper insisted Canada would only sign a deal that was in the country's best interests, noting it would also have to be ratified by Parliament. Harper also promised to release details of the pact, but Mulcair was skeptical.

"You ask anybody in Canada who's ever dealt with Stephen Harper on these things and the answer is going to be no," Mulcair said.

Harper has previously indicated the trade agreement could cause the auto industry some pain.

In this handout provided to journalists covering Tom Mulcair's election campaign tour, his planned itinerary across Ontario is shown, spotlighting places where the Trans-Pacific Partnership trade deal may be of concern. (NDP handout)

Mulcair headed to Ontario Tory strongholds

Mulcair, who has hammered the theme of job losses throughout the campaign, returned to the topic Saturday, saying 30,000 family farms and 400,000 good manufacturing jobs have disappeared on Harper's watch.

To stress that point, Mulcair was headed after the farm visit to Conservative strongholds in southwestern Ontario, where he was expected to make a six-riding blitz on Sunday. The area has been hit by the decline in manufacturing, especially in the important auto industry.

The NDP's electoral fortunes rest heavily on maintaining the kind of surge in Quebec that propelled the party to official Opposition status in 2011.

However, polls suggest Quebecers are less enthralled with Mulcair this time around, in part because of his view that women should be allowed to wear veils at citizenship ceremonies.

A party insider said Mulcair had tackled the niqab issue head on, but said they would have pursued their long-standing support for supply management regardless.

The CBC's James Fitz-Morris explains what exactly the Trans-Pacific Partnership could mean for Canada. Could it be a boon for the economy? 1:36

The farm visit came in the riding of Saint-Hyacinthe-Bagot, which the New Democrats took from the Bloc Quebecois in the last vote as part of the "orange crush."

However, the incumbent is not running this time, putting the riding into play for the Bloc, which under Gilles Duceppe is trying to regain the seats it lost in 2011.