ATLANTA – A last-minute sprint toward a historic trade agreement has turned into yet another marathon negotiating session, as a few lingering issues including Canadian dairy have repeatedly delayed a deal announcement Sunday.

As a result a planned news conference to announce the deal was rescheduled from 4 p.m., to 6 p.m., then 8 p.m., and is now pushed to a time to be determined, in a fitting finale to a ministerial meeting marked by all-night negotiations that was supposed to last two days, then three, then four and is now in its fifth and probably final day.

It should become clear within hours whether these talks conclude under Canada’s current government, during the election campaign, or under the post-election government led by the Conservative party or one of its rivals. Whatever happens, the deal would still need to be approved by the next Parliament.

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Stephen Harper had planned for a quiet day off the election trail but it wound up consumed by trade talks, with the prime minister and Conservative leader getting briefing in Ottawa from the negotiating team in Atlanta.

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The dynamics delaying the Trans-Pacific Partnership deal were explained by one of the trade ministers involved the 12-country talks. He said a struggle over next-generation pharmaceuticals has had a cascading effect on attempts to resolve other issues.

“Look, it’s not done yet,” said Australia’s Andrew Robb.

He explained that the U.S. and Australia had worked all night to resolve their differences on cutting-edge, cell-based medicines and made a breakthrough around 3 a.m.

He said they’d succeeded at establishing a model that bridges the gap between two entrenched positions: the more business-friendly, eight-year patent-style protections the U.S. wants for biologics, and the more patient-and-taxpayer-friendly five-year model preferred by Australia and others. But that caused an uneven ripple effect. Some other countries weren’t pleased with the compromise, and now that discussion has become more multi-sided with two or three holdouts remaining, he said. Canada’s not too involved in that skirmish. But the delay, according to Robb, wound up pushing other issues to the backburner until Sunday morning and they’re still being worked out. Insiders say access to Canadian grocery shelves is chief among them. Negotiators have been haggling about how much foreign butter, condensed milk and other dairy products should be allowed into Canada. New Zealand helped create the TPP project a decade ago and it wants to sell more butter in North America — especially in the United States. It says the U.S., however, won’t open its own agriculture sector until getting some assurance that American producers could sell more in Canada and Mexico.

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