NDP Leader Tom Mulcair hit back at Stephen Harper today, calling him "weak" and "vulnerable" as Canada continues to negotiate on the Trans-Pacific Partnership trade deal, after Canada and 11 other countries failed to reach an agreement before the election was called on Sunday.

This was the first time, since the election campaign was launched on Sunday, that Mulcair took questions from reporters. On Monday, Mulcair was said to be preparing for the first leaders' debate this Thursday.

"What's going to be on the table with Mr. Harper negotiating that right in the middle of an election campaign? He's weak, he's vulnerable, he was never a very good negotiator to begin with, but we're concerned about very important subjects," Mulcair said during his first question-and-answer session in Montreal, on day 3 of the federal election campaign.

The Trans-Pacific Partnership would stretch from Japan to Chile and represent almost 40 per cent of the world GDP, but as talks hit a snag, New Zealand made it clear it would not accept a deal that does not significantly open dairy markets.

While the Conservatives have often accused the NDP of being against trade, Mulcair said the NDP was "enthusiastically in favour" of a TPP deal, but had some concerns about Canada's supply management system.

"Supply management is something that has allowed Canadian farming families to hold on to their farms, despite the ups and downs … and lots of those farmers are worried.

"We've met them in southern Ontario, we met them here in Quebec, and we're going to stand up strongly and defend every step of the way our supply management system," said Mulcair.

The NDP leader said his party had voted in favour of the Canada-Korea Free Trade Agreement, and while his party is in favour of an agreement with the European Union, he had tough questions about the fine print.

"There's still no text," Mulcair said of the Canada and European Union Comprehensive Economic and Trade Agreement (CETA) deal. "The details actually matter on these things."

Harper, earlier on Tuesday, pledged to reintroduce a home renovation tax credit if re-elected on Oct. 19, but with a budget already stretched thin, Harper said it would not return right away.

Mulcair said Canadians would judge Harper on his 10-year record and not on last-minute promises.

"Now at the 11th hour, in the middle of an election campaign, he's discovered the merits of something he got rid of," Mulcair said, adding that "Canadians will judge him on what he actually did, not on what he is promising during an election campaign."

Trudeau, Mulcair trade barbs

Asked for his reaction to Liberal Leader Justin Trudeau calling some of the NDP promises "a mirage," Mulcair hit back, saying that at least his party has a plan to present to Canadians.

"I'm not sure what that word is supposed to mean, but maybe he can start sharing with us his plan, because nobody knows it," Mulcair said.

The NDP leader also accused Trudeau of endorsing trade deals, such as CETA, without seeing the details first.

He also touted his party's opposition to Bill C-51, the Anti-terrorism Act, which the Liberals supported but said they would amend if forming government after the next election.

Mulcair defended his party's promise to slowly increase the corporate tax rate against repeated attacks from the Conservatives that it would wreck the economy. Increasing the tax rate on large corporations, the NDP leader said, would pay for other promises such as a national child daycare program.

On the subject of key energy projects, Mulcair said the Conservative leader only had himself to blame for projects like the Keystone XL pipeline failing to launch. "Mr. Harper has scrapped so much environmental legislation that the public said they can't have faith in that anymore,"

Mulcair said. "Mr. Harper's rip-and-ship approach simply isn't working, like so much else that he's done. He thought he could bully his way to yes with regard to a lot of these projects ... that's not working so well."

The NDP leader was in Mont-Royal on Tuesday where he first announced his intention to run federally eight years ago alongside then NDP leader Jack Layton. Harper made Mont-Royal his first campaign stop after the election was launched on Sunday.