It's been nearly three decades since Canadians fought an election over trade.

The 1988 campaign, dominated by the Canada-U.S. free-trade agreement, put the country on a path toward more open borders.

Once on opposite sides of the debate, the Conservatives and Liberals are now unabashedly pro-free trade.

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Even the NDP, a party with deep anti-trade roots, has moved cautiously toward the political middle on the issue. It endorsed the recent free-trade pact with South Korea and welcomed the completion of negotiations with Europe on the pending Comprehensive Economic and Trade Agreement (CETA).

But the NDP has been conspicuously vague about where it stands on the Trans-Pacific Partnership – a massive free-trade zone spanning a dozen Pacific Rim countries with a combined economic output of $28-trillion (U.S.).

The TPP is no longer an abstraction. U.S. President Barack Obama secured "fast-track" trade negotiating powers from Congress this week after a protracted political battle, clearing the way for the possible completion of TPP negotiations before the Canadian federal election in October.

The deal could now emerge as the sleeper issue in the campaign.

Prime Minister Stephen Harper dispelled any lingering doubts about his commitment to the deal, saying Thursday that joining the TPP is "essential" for the Canadian economy.

The Liberals also back the TPP. But Liberal Leader Justin Trudeau called this week for re-engaging with often-ignored NAFTA partner Mexico and repairing strained relations with the United States, the destination for roughly three-quarters of Canada's exports.

NDP Leader Thomas Mulcair has been much harder to pin down. In a major economic speech earlier in June, Mr. Mulcair talked about boosting exports, but never once mentioned the TPP or other trade agreements.

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The NDP's platform and its track record suggest an agnostic attitude toward these deals. The party's decisions suggest a more protectionist bent than either the Liberals or the Conservatives.

The NDP's official platform emphasizes "fair trade" rather than "free trade," suggesting highly qualified support for trade agreements. The party opposes investor-state provisions, which allow companies to sue governments for unfair treatment, even though such rules are an integral part of NAFTA, CETA and the TPP.

The NDP also wants to renegotiate NAFTA to better protect Canada's investment and energy security. The party platform talks of "regulating the flow of international capital" and preserving the right of municipalities and provinces to continue steering contracts to local suppliers.

These positions put the NDP at odds with elements of both CETA and the TPP. The deals will open up vast areas of government purchasing, improving the access of Canadian companies to local government contracts in Europe, and vice versa.

The NDP also has a seemingly contradictory position on two persistent trade irritants with the United States: Buy American purchasing rules and a U.S. country-of-origin meat labelling law, recently declared illegal by the World Trade Organization because it treats Canadian and Mexican meat unfairly.

The party has been highly critical of these protectionist policies. And yet, senior NDP MPs have backed legislation that would impose similar measures in Canada. For example, NDP trade critic Don Davies introduced a country-of-origin food labelling law in 2009 that would have covered all food and ingredients. Also in 2009, NDP Opposition House Leader Peter Julian introduced a bill to "give preference" to Canadian products in contracts paid for with federal transfers to provinces and municipalities.

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The NDP is not alone in having awkward trade policy inconsistencies. All three parties continue to defend the supply management system, which shields Canadian dairy, chicken and egg producers from foreign competition with prohibitive tariffs.

And Mr. Harper reiterated that support this week, insisting the government is "working to protect our system of supply management," even as it pursues entry into the TPP.

The good news for voters is that a TPP deal would force all three parties to lay their cards on the table, prior to the election.

The government must now decide how far it's willing to go to defend dairy and chicken farmers.

For the NDP, the choice is to stand with the government or make trade a wedge issue.