A last-minute sprint toward a historic trade agreement turned into yet another marathon negotiating session, as a few lingering issues including Canadian dairy repeatedly delayed an announcement of a deal 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 then indefinitely, 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.

Journalists were summoned on short notice to the downtown Ottawa building housing the foreign affairs and trade department for a confidential briefing at 3 p.m. ET.

But coffee grew cold waiting for that briefing to start. The only explanation offered the media and industry stakeholders who had rushed over was that things in Ottawa couldn't proceed until the final offers in Atlanta were sorted out.

After a while it became evident the deal wasn't done after all.

Pizza arrived but the promised briefing materials did not. Word finally came that an announcement in Atlanta could take until midnight or even later. Officials sent people home, with promises to phone anyone interested in an overnight wake up call if, in fact, agreement is reached in the wee hours.

Stephen Harper had planned for a quiet day off the election trail but it ended up consumed by trade talks, with the prime minister in Ottawa getting phone briefings from the negotiating team in Atlanta. Harper will speak on the deal in the morning if one is reached overnight.

'It's not done yet'

The talks appear likely to break up Monday as some ministers planned to leave for a G20 summit. Japan's envoy has warned he can't stick around through the day.

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 is 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.

Haggling about foreign butter

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.

Currently, 90 per cent of the Canadian dairy market is closed to foreign products. The system allows for stable incomes in farming communities, but it limits options and drives up prices at the grocery store.

Representatives of the dairy lobby milled about the convention site late Sunday. They professed to be still in the dark about what market-access offer Canada had made.

Reduce or eliminate barriers

The agreement would reduce or eliminate barriers in a wide range of sectors and could lead to more Canadian exports of pork, beef, canola, high-tech machinery and a variety of other products.

It would also entrench new international trade standards in Asia, setting a template should any other countries in that fast-growing region — like China — want to join.

Other parts could be controversial in Canada. It's expected to increase imports of foreign car parts and possibly dairy, which could mean lower prices and greater selection for consumers but also hurt some workers in both sectors.

Cue the political debates.

The NDP's opposition to the TPP process is an early example of the political challenges it could face in several countries as it is voted on by their parliaments.

"Just know this. If Mr. Harper's secret deal does not protect supply management in its entirety, if it does not protect our manufacturing sector, if it does not protect your ability to buy your pharmaceuticals and your prescription drugs at a decent price the NDP will not feel bound by Mr. Harper's secret deal," NDP Leader Tom Mulcair said in Sarnia, Ont. today.

Following a campaign rally in Brampton, Ont., Liberal Party Leader Justin Trudeau said his party is a "pro-trade party" and that they will look at the details of the deal.

"The fact is the prime minister has continued to say he has protected Canadian interests, including supply management, of course the problem is that he has been secretive and non-transparent in this and we need to make sure that we are actually indeed creating a trade deal that is good for Canadians," Trudeau said.

The biggest potential test would come in a few months, as U.S. Congress votes on the deal and conflicting pressures from the political left and right threaten to make that vote a nailbiter.

It's unclear when the public might see the fine print — and whether it would be available before Canadians head to the polls Oct. 19. One of the outstanding sources of uncertainty is when a legal review might be completed of the actual text of the deal.