In second place two weeks before Canada’s election, the Conservative prime minister, Stephen Harper, praised a “historic” accord creating the world’s largest free trade area, saying access to Japan would be a boon for Canadian farmers and resource sectors.

Harper touted the Trans-Pacific Partnership as a good accord that offers only limited access to Canada’s politically important farm industry, largely neutralizing an attack from the left-leaning New Democratic party, which has been sinking in polls.

The NDP leader, Thomas Mulcair, derided Harper for backing the 12-nation agreement in the midst of a campaign when the government is supposed to be in a neutral caretaker status.

“It’s inconceivable that Stephen Harper signed a secret deal in the midst of an election campaign. He’s sacrificing our family farms, he is selling out auto workers,” Mulcair said.

Mulcair vowed to stand up against the deal but stopped short of saying he would back out of it if he won the election.

Previously Mulcair had said he would not be bound by the terms of the deal. The NDP’s last hope for political victory may be voter anger over free trade.

The centrist Liberal Party, which has edged ahead of Harper in recent days, signalled it would probably support the deal.

Harper told a news conference: “You are either in or out … We choose to be in because there is simply too much to gain for Canada.”

Harper said that under the agreement, Canadian exporters would gain almost tariff-free access to nearly 800 million consumers in the Asia-Pacific region.

“Crucially for us, Japan [represents] a vast new market for Canadian enterprise and goods,” he argued, saying the agreement “exceeded my best expectations”.

A Nanos poll released on Sunday suggested the race to the 19 October election, which had been a virtual three-way tie for the first two months of the 11-week campaign, was increasingly between Harper and the Liberal leader, Justin Trudeau. The Liberals were winning 35% support, with the Conservatives at 31% and the NDP trailing at 24%.

While farmers had said they could be crippled if Canada gave up too much of its supply management system that keeps domestic dairy and poultry prices high, Harper said the deal offered access to just 3.25% of the dairy market and about 2% of the poultry market over five years.

In a move to shore up support among voters in the Conservative heartland, Harper announced compensation to farmers for losses they might suffer.

For voters in industrial Ontario, the province with the most seats up for grabs, he said the Conservatives would soon announce measures to attract new auto investment.