Apart from balancing the budget, the third Harper government has considered trade its highest priority. That's why Stephen Harper is poised to toss accepted electoral wisdom aside and sign the landmark Trans-Pacific Partnership trade agreement.

The agreement, which will transform Canada's trade relationship with Asian and Pacific countries, will signal the death knell for dairy and poultry subsidies and shift the electoral calculus in Quebec, Ontario and British Columbia.

Conservative insiders acknowledge that the decision is likely to cost the party seats in rural Ontario on election night in October. But Mr. Harper is determined to commit Canada to the accord regardless. This is his legacy.

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The government has already negotiated and signed an ambitious agreement with the European Union. Now, with congressional hurdles overcome, the United States is ready, along with Japan, Canada and nine other countries, to proceed with the Trans-Pacific Partnership (TPP), the most ambitious regional trade agreement ever negotiated, encompassing 40 per cent of global GDP.

Because Mexico is also a member, the new accord will supplant the North American Free Trade Agreement. Staying outside TPP, Mr. Harper has argued in government meetings, would see the U.S. and Mexico move to a new level of trading integration, with Canada left behind.

And being part of TPP will, at a stroke, vastly expand Canada's economic links with major developed and developing Pacific countries such as Japan, Malaysia and Vietnam.

It is possible to envision a not-too-distant future in which most countries of the Atlantic and Pacific regions become part of a single, global trade zone.

But in Canada, TPP will be no easy sell. Although there will doubtless be a phase-in period, along with compensation for affected farmers, signing means the end of supply management, which for decades has protected the dairy and poultry industries.

Those farmers will be justifiably angry at the Conservatives. Mr. Harper promised he could get a TPP deal while still protecting supply management. In the end, he decided to sacrifice the interests of the farmers to the greater interest.

Rural seats in Southwestern and Eastern Ontario that were once considered safe will now be very much in play.

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The Conservatives will fight hard to hold those seats regardless, stressing the crucial link between rural values and Conservative priorities. And Mr. Harper will trumpet the accord in places such as the Greater Toronto Area and B.C.'s Lower Mainland.

Signing the TPP, he will say, will bring Canada's trade relationships into the 21st century, accelerate our economic realignment from the Atlantic to the Pacific and offer further proof that only the Conservatives can be trusted to manage the national economy.

Ironically, Mr. Harper's decision is likely to be good news for Thomas Mulcair. Many of Canada's dairy farms are in Quebec, in ridings held by the NDP. If, as expected, the New Democrats oppose the accord because it fails to protect supply management, Mr. Mulcair will be able to tell rural Quebec voters that only his party truly represents their interests.

For Justin Trudeau, the situation is problematic. The Liberals place a high priority in taking francophone ridings from the NDP in Quebec, which would incline the party toward opposing the deal. But the Liberals are also determined to show middle-class voters in the rest of Canada that they can be trusted to protect the country's economic interests. How could these interests be served by keeping Canada outside the world's most important regional trade agreement?

This is a point the Conservatives can be expected to make – forcefully – if Mr. Trudeau decides to join Mr. Mulcair in opposing the TPP.

Politics as well as policy, then, will inform the debate over the TPP. In an election season, with economic trust a key concern, that debate will be intense.